diff --git a/CHANGELOG.md b/CHANGELOG.md
index 618cf10..9c34104 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,7 @@
 # main ([unreleased](https://github.com/fastruby/rails_stats/compare/v2.1.0...main))
 
+* [FEATURE: Support component-based and Packwerk (packs) applications with a per-pack and Core breakdown](https://github.com/fastruby/rails_stats/issues/23)
+* [BUGFIX: Respect an explicit path argument (e.g. `rake stats[packs/pack1]`) when running inside a booted Rails app](https://github.com/fastruby/rails_stats/issues/23)
 * [CHORE: Improve the GH Test Workflow](https://github.com/fastruby/rails_stats/pull/35)
 * [BUGFIX: Explicitly set format as text for `Bundler::Stats::CLI` on `ConsoleFormatter`](https://github.com/fastruby/rails_stats/pull/43)
 
diff --git a/README.md b/README.md
index e94c4fb..ca3bdf2 100644
--- a/README.md
+++ b/README.md
@@ -71,6 +71,56 @@ Directory: /path/to/app/
 $ for dir in /path/to/many/engines/*/; do bundle exec rake stats[$dir]; done
 ```
 
+### Component-based and Packwerk (packs) applications
+
+RailsStats understands large Rails monoliths that are organized into
+[packs](https://github.com/Shopify/packwerk) (via
+[packs-rails](https://github.com/rubyatscale/packs-rails)) or
+[components](https://cbra.info/) (component-based Rails).
+
+When packs or components are detected, `rake stats` automatically prints a
+breakdown per pack/component, a `Core Application` section for everything that
+lives in the main app, and a `Total (all packs)` section that aggregates the
+whole monolith:
+
+```bash
+$ bundle exec rake stats
+
+== Core Application ==
++----------------------+---------+---------+---------+---------+---------+-----+-------+
+| Name                 | Files   | Lines   |     LOC | Classes | Methods | M/C | LOC/M |
++----------------------+---------+---------+---------+---------+---------+-----+-------+
+| ...                                                                                 |
++----------------------+---------+---------+---------+---------+---------+-----+-------+
+
+== Pack: packs/pack1 ==
++----------------------+---------+---------+---------+---------+---------+-----+-------+
+| ...                                                                                 |
++----------------------+---------+---------+---------+---------+---------+-----+-------+
+
+== Total (all packs) ==
++----------------------+---------+---------+---------+---------+---------+-----+-------+
+| ...                                                                                 |
++----------------------+---------+---------+---------+---------+---------+-----+-------+
+```
+
+A directory is treated as a pack when it contains a `package.yml` file (the
+Packwerk marker) or when it lives directly under a `packs/` or `components/`
+directory and holds any of the usual code folders (`app`, `lib`, `spec`,
+`test`). Nested packs (`packs/a/packs/b`) are supported.
+
+You can also scope stats to a single pack, even from inside a booted Rails app:
+
+```bash
+$ bundle exec rake stats[packs/pack1]
+```
+
+The same breakdown is available in JSON under a `"packs"` key:
+
+```bash
+$ bundle exec rake stats[.,json]
+```
+
 ### Within your Rails project
 
 You can also include it within your Rails application to _replace_ the default `rake stats` implementation.
diff --git a/lib/rails_stats/all.rb b/lib/rails_stats/all.rb
index 6894c1c..d664590 100644
--- a/lib/rails_stats/all.rb
+++ b/lib/rails_stats/all.rb
@@ -1,9 +1,17 @@
+module RailsStats
+  # Vendored / generated directory names we never descend into or treat as a
+  # pack. Shared by PackFinder (which builds IGNORED_PATH from it) and Util
+  # (which matches them by basename while walking the tree).
+  IGNORED_DIRS = %w[node_modules vendor tmp log .git].freeze
+end
+
 require 'rails_stats/stats_calculator'
 require 'rails_stats/stats_formatter'
 require 'rails_stats/json_formatter'
 require 'rails_stats/console_formatter'
 require 'rails_stats/inflector'
 require 'rails_stats/code_statistics_calculator'
+require 'rails_stats/pack_finder'
 require 'rails_stats/util'
 require 'rails_stats/app_statistics'
 require 'rails_stats/test_statistics'
diff --git a/lib/rails_stats/app_statistics.rb b/lib/rails_stats/app_statistics.rb
index 05eaf80..bdedd3d 100644
--- a/lib/rails_stats/app_statistics.rb
+++ b/lib/rails_stats/app_statistics.rb
@@ -1,6 +1,6 @@
 module RailsStats
   class AppStatistics
-    attr_reader :statistics, :total, :test
+    attr_reader :statistics, :total, :test, :directory
 
     def initialize(directory)
       @directories = []
diff --git a/lib/rails_stats/console_formatter.rb b/lib/rails_stats/console_formatter.rb
index 83b56d0..a2d5cbd 100644
--- a/lib/rails_stats/console_formatter.rb
+++ b/lib/rails_stats/console_formatter.rb
@@ -5,16 +5,10 @@ class ConsoleFormatter < StatsFormatter
     def to_s
       Bundler::Stats::CLI.start(['--format', 'text'])
 
-      print_header
-      sorted_keys = @statistics.keys.sort
-      sorted_keys.each { |key| print_line(key, @statistics[key]) }
-      print_splitter
-
-      if @grand_total
-        print_line("Code", @code_total)
-        print_line("Tests", @tests_total)
-        print_line("Total", @grand_total)
-        print_splitter
+      if calculator.packs?
+        print_grouped
+      else
+        print_flat
       end
 
       print_code_test_stats
@@ -24,6 +18,44 @@ def to_s
 
     private
 
+      def print_flat
+        print_table(@statistics)
+      end
+
+      def print_grouped
+        @statistics_by_group.each do |group, group_statistics|
+          puts "== #{group_label(group)} =="
+          print_table(group_statistics)
+          puts ""
+        end
+
+        puts "== #{grand_total_label} =="
+        print_table(@statistics)
+      end
+
+      def group_label(group)
+        group == RailsStats::StatsCalculator::CORE_GROUP ? "Core Application" : "Pack: #{group}"
+      end
+
+      def grand_total_label
+        "Total (all packs)"
+      end
+
+      # Prints a full table (header, one line per concept, and Code/Tests/Total
+      # totals) for the given {concept => CodeStatisticsCalculator} hash.
+      def print_table(statistics)
+        code, tests, grand = calculator.totals_for(statistics)
+
+        print_header
+        statistics.keys.sort.each { |key| print_line(key, statistics[key]) }
+        print_splitter
+
+        print_line("Code", code)
+        print_line("Tests", tests)
+        print_line("Total", grand)
+        print_splitter
+      end
+
       def print_header
         print_splitter
         puts "| Name                 | Files   | Lines   |     LOC | Classes | Methods | M/C | LOC/M |"
diff --git a/lib/rails_stats/cucumber_statistics.rb b/lib/rails_stats/cucumber_statistics.rb
index 2856073..2c78778 100644
--- a/lib/rails_stats/cucumber_statistics.rb
+++ b/lib/rails_stats/cucumber_statistics.rb
@@ -1,6 +1,6 @@
 module RailsStats
   class CucumberStatistics
-    attr_reader :statistics, :total, :test
+    attr_reader :statistics, :total, :test, :directory
 
     def initialize(directory)
       @test = true
diff --git a/lib/rails_stats/gem_statistics.rb b/lib/rails_stats/gem_statistics.rb
index fbc706b..61887ad 100644
--- a/lib/rails_stats/gem_statistics.rb
+++ b/lib/rails_stats/gem_statistics.rb
@@ -1,6 +1,6 @@
 module RailsStats
   class GemStatistics
-    attr_reader :statistics, :total, :test
+    attr_reader :statistics, :total, :test, :directory
 
     def initialize(directory)
       @test = false
diff --git a/lib/rails_stats/json_formatter.rb b/lib/rails_stats/json_formatter.rb
index cb1f1d2..afd17ad 100644
--- a/lib/rails_stats/json_formatter.rb
+++ b/lib/rails_stats/json_formatter.rb
@@ -17,6 +17,8 @@ def result
       @result << stat_hash("Tests", @tests_total).merge(code_test_hash) if @tests_total
       @result << stat_hash("Total", @grand_total).merge(code_test_hash) if @grand_total
 
+      @result << { "packs" => packs_breakdown } if calculator.packs?
+
       @result << { "schema_stats" => schema_info }
       @result << { "polymorphic_stats" => print_polymorphic_stats }
 
@@ -29,6 +31,22 @@ def to_s
 
     private
 
+      # Per-pack / per-component breakdown, keyed by group name ("Core" plus
+      # each pack's relative path). Each group lists its concept stats followed
+      # by Code/Tests/Total rows for that group.
+      def packs_breakdown
+        @statistics_by_group.each_with_object({}) do |(group, group_statistics), out|
+          code, tests, grand = calculator.totals_for(group_statistics)
+
+          rows = group_statistics.map { |key, stats| stat_hash(key, stats) }
+          rows << stat_hash("Code", code).merge("total" => true)
+          rows << stat_hash("Tests", tests).merge("total" => true)
+          rows << stat_hash("Total", grand).merge("total" => true)
+
+          out[group] = rows
+        end
+      end
+
       def code_test_hash
         code  = calculator.code_loc
         tests = calculator.test_loc
diff --git a/lib/rails_stats/pack_finder.rb b/lib/rails_stats/pack_finder.rb
new file mode 100644
index 0000000..e9ffbc7
--- /dev/null
+++ b/lib/rails_stats/pack_finder.rb
@@ -0,0 +1,67 @@
+module RailsStats
+  # Detects "packs" (packwerk / packs-rails) and "components" (component-based
+  # Rails, à la Stephan Hagemann) inside a Rails application so that stats can
+  # be reported per pack/component plus a "Core" group for everything else.
+  #
+  # A directory is considered a pack when either:
+  #
+  #   * it contains a `package.yml` file (the packwerk marker), or
+  #   * it is a direct child of a `packs/` or `components/` directory and holds
+  #     any of the usual code folders (`app`, `lib`, `spec`, `test`).
+  #
+  # Nested packs (e.g. `packs/a/packs/b`) are supported. The root directory
+  # itself is never reported as a pack.
+  module PackFinder
+    extend self
+
+    CODE_FOLDERS   = %w[app lib spec test].freeze
+    PACK_CONTAINERS = %w[packs components].freeze
+
+    # Path regex used to filter glob results to skip vendored/generated
+    # directories. Built from the shared RailsStats::IGNORED_DIRS list.
+    IGNORED_PATH   = %r{/(#{IGNORED_DIRS.map { |dir| Regexp.escape(dir) }.join("|")})/}.freeze
+
+    # Returns the absolute paths of every detected pack/component, sorted.
+    def find(root_directory)
+      root = File.absolute_path(root_directory)
+      packs = {}
+
+      collect_packwerk_packs(root, packs)
+      collect_convention_packs(root, packs)
+
+      packs.keys.sort
+    end
+
+    private
+
+    def collect_packwerk_packs(root, packs)
+      Dir.glob(File.join(root, "**", "package.yml")).each do |marker_path|
+        next if marker_path =~ IGNORED_PATH
+
+        dir = File.absolute_path(File.dirname(marker_path))
+        next if dir == root
+
+        packs[dir] = true
+      end
+    end
+
+    def collect_convention_packs(root, packs)
+      PACK_CONTAINERS.each do |container|
+        Dir.glob(File.join(root, "**", container, "*")).each do |path|
+          next if path =~ IGNORED_PATH
+          next unless File.directory?(path)
+
+          dir = File.absolute_path(path)
+          next if dir == root
+          next unless code_folder?(dir)
+
+          packs[dir] = true
+        end
+      end
+    end
+
+    def code_folder?(dir)
+      CODE_FOLDERS.any? { |folder| File.directory?(File.join(dir, folder)) }
+    end
+  end
+end
diff --git a/lib/rails_stats/root_statistics.rb b/lib/rails_stats/root_statistics.rb
index 35b5536..a8b9d79 100644
--- a/lib/rails_stats/root_statistics.rb
+++ b/lib/rails_stats/root_statistics.rb
@@ -1,6 +1,6 @@
 module RailsStats
   class RootStatistics
-    attr_reader :statistics, :total, :test
+    attr_reader :statistics, :total, :test, :directory
 
     ROOT_FOLDERS = {
       "lib" => "Libraries",
diff --git a/lib/rails_stats/spec_statistics.rb b/lib/rails_stats/spec_statistics.rb
index f8edda3..1523233 100644
--- a/lib/rails_stats/spec_statistics.rb
+++ b/lib/rails_stats/spec_statistics.rb
@@ -1,6 +1,6 @@
 module RailsStats
   class SpecStatistics
-    attr_reader :statistics, :total, :test
+    attr_reader :statistics, :total, :test, :directory
 
     SPEC_FOLDERS = ['controllers',
                     'features',
diff --git a/lib/rails_stats/stats_calculator.rb b/lib/rails_stats/stats_calculator.rb
index f8c283e..0c7ac25 100644
--- a/lib/rails_stats/stats_calculator.rb
+++ b/lib/rails_stats/stats_calculator.rb
@@ -8,12 +8,14 @@ class StatsCalculator
                          'assets']
 
     def initialize(root_directory)
-      @root_directory = root_directory
+      @root_directory = File.absolute_path(root_directory)
       @schema_path    = File.join(@root_directory, "db", "schema.rb")
       @structure_path = File.join(@root_directory, "db", "structure.sql")
+      @pack_roots     = PackFinder.find(@root_directory)
       @key_concepts   = calculate_key_concepts
       @projects       = calculate_projects
       @statistics     = calculate_statistics
+      @statistics_by_group = calculate_statistics_by_group
       @code_loc       = calculate_code
       @test_loc       = calculate_tests
       @schema         = calculate_create_table_calls
@@ -21,7 +23,31 @@ def initialize(root_directory)
       @files_total, @code_total, @tests_total, @grand_total = calculate_totals
     end
 
-    attr_reader :code_loc, :code_total, :files_total, :grand_total, :statistics, :test_loc, :tests_total, :schema, :schema_path, :structure_path, :polymorphic
+    attr_reader :code_loc, :code_total, :files_total, :grand_total, :statistics, :test_loc, :tests_total, :schema, :schema_path, :structure_path, :polymorphic, :pack_roots, :statistics_by_group
+
+    CORE_GROUP = "Core".freeze
+
+    # Returns true when the application is split into packs/components, i.e.
+    # there is at least one group beyond the core application.
+    def packs?
+      @pack_roots.any?
+    end
+
+    # Given a {concept => CodeStatisticsCalculator} hash (such as one of the
+    # values in +statistics_by_group+ or the flat +statistics+), returns the
+    # [code, tests, grand] totals for that group.
+    def totals_for(group_statistics)
+      code  = CodeStatisticsCalculator.new
+      tests = CodeStatisticsCalculator.new
+      grand = CodeStatisticsCalculator.new
+
+      group_statistics.each_value do |stats|
+        grand.add(stats)
+        stats.test ? tests.add(stats) : code.add(stats)
+      end
+
+      [code, tests, grand]
+    end
 
     private
 
@@ -83,7 +109,23 @@ def calculate_test_projects
       end
 
       def calculate_root_projects
-        [RootStatistics.new(@root_directory)]
+        projects = [RootStatistics.new(@root_directory)]
+        projects + calculate_pack_root_projects
+      end
+
+      # Packs that are not self-contained gems/engines (no gemspec) still have
+      # a `lib` (and sometimes `config`) directory that would otherwise be
+      # missed, since only the root RootStatistics scans the top-level lib.
+      # Gems are already covered by GemStatistics, so skip them here to avoid
+      # double-counting.
+      def calculate_pack_root_projects
+        @pack_roots.reject { |pack_root| gem_pack?(pack_root) }.collect do |pack_root|
+          RootStatistics.new(pack_root)
+        end
+      end
+
+      def gem_pack?(pack_root)
+        Dir[File.join(pack_root, "*.gemspec")].any?
       end
 
       def calculate_cucumber_projects
@@ -104,6 +146,62 @@ def calculate_statistics
         out
       end
 
+      # Same aggregation as +calculate_statistics+, but keeps each project's
+      # numbers grouped by the pack/component it belongs to (or "Core"). The
+      # returned hash is ordered with "Core" first followed by packs sorted by
+      # their relative path.
+      def calculate_statistics_by_group
+        grouped = {}
+        @projects.each do |project|
+          group = group_key_for(project.directory)
+          grouped[group] ||= {}
+          project.statistics.each do |key, stats|
+            grouped[group][key] ||= CodeStatisticsCalculator.new(project.test)
+            grouped[group][key].add(stats)
+          end
+        end
+
+        # Drop empty concepts so a pack with, say, only models does not list a
+        # bunch of zero-line rows.
+        grouped.each_value do |group_statistics|
+          group_statistics.delete_if { |_key, stats| stats.lines == 0 }
+        end
+        grouped.delete_if { |_group, group_statistics| group_statistics.empty? }
+
+        sort_groups(grouped)
+      end
+
+      def sort_groups(grouped)
+        ordered = {}
+        ordered[CORE_GROUP] = grouped[CORE_GROUP] if grouped.key?(CORE_GROUP)
+        grouped.keys.reject { |key| key == CORE_GROUP }.sort.each do |key|
+          ordered[key] = grouped[key]
+        end
+        ordered
+      end
+
+      # Maps a project directory to its group: the relative path of the pack it
+      # lives in, or "Core" when it belongs to the main application.
+      def group_key_for(directory)
+        pack_root = pack_root_for(directory)
+        return CORE_GROUP unless pack_root
+
+        relative = pack_root.sub(/\A#{Regexp.escape(@root_directory)}\/?/, "")
+        relative.empty? ? CORE_GROUP : relative
+      end
+
+      # Returns the most specific (longest matching) pack root that contains the
+      # given directory, or nil when it is not part of any pack.
+      def pack_root_for(directory)
+        directory = File.absolute_path(directory.to_s)
+        best = nil
+        @pack_roots.each do |pack_root|
+          next unless directory == pack_root || directory.start_with?("#{pack_root}/")
+          best = pack_root if best.nil? || pack_root.length > best.length
+        end
+        best
+      end
+
       def calculate_totals
         files_total = @statistics.sum do |k,value|
           value.files_total
diff --git a/lib/rails_stats/stats_formatter.rb b/lib/rails_stats/stats_formatter.rb
index f03760a..0e386bf 100644
--- a/lib/rails_stats/stats_formatter.rb
+++ b/lib/rails_stats/stats_formatter.rb
@@ -3,6 +3,7 @@ class StatsFormatter
     def initialize(calculator, opts = {})
       @calculator  = calculator
       @statistics  = calculator.statistics
+      @statistics_by_group = calculator.statistics_by_group
       @code_total  = calculator.code_total
       @tests_total = calculator.tests_total
       @grand_total = calculator.grand_total
diff --git a/lib/rails_stats/tasks.rb b/lib/rails_stats/tasks.rb
index 067aff5..df3a9ba 100644
--- a/lib/rails_stats/tasks.rb
+++ b/lib/rails_stats/tasks.rb
@@ -3,8 +3,11 @@
   Rake::Task["stats"].clear # clear out normal one if there
   require 'rails_stats/all'
 
+  # Respect an explicitly given path even when running inside a booted Rails
+  # app. Only fall back to Rails.root when no path was provided. This is what
+  # makes `rake stats[packs/pack1]` work from inside the app (see issue #23).
   path = args[:path]
-  path = Rails.root.to_s if defined?(Rails)
+  path ||= Rails.root.to_s if defined?(Rails)
   fmt = args[:format] || ""
   raise "no path given for stats" unless path
 
diff --git a/lib/rails_stats/test_statistics.rb b/lib/rails_stats/test_statistics.rb
index 927a998..c7f57eb 100644
--- a/lib/rails_stats/test_statistics.rb
+++ b/lib/rails_stats/test_statistics.rb
@@ -1,6 +1,6 @@
 module RailsStats
   class TestStatistics
-    attr_reader :statistics, :total, :test
+    attr_reader :statistics, :total, :test, :directory
 
     SPEC_FOLDERS = ['controllers',
                     'functional',
diff --git a/lib/rails_stats/util.rb b/lib/rails_stats/util.rb
index f9baf45..ad93fde 100644
--- a/lib/rails_stats/util.rb
+++ b/lib/rails_stats/util.rb
@@ -81,8 +81,16 @@ def calculate_directory_statistics(directory, pattern = /.*\.(rb|js|jsx|ts|tsx|c
       Dir.foreach(directory) do |file_name|
         path = "#{directory}/#{file_name}"
 
-        if File.directory?(path) && (/^\./ !~ file_name)
+        # Skip vendored / generated directories (e.g. a pack's
+        # app/webpack/node_modules). Pruning here avoids both the wasted walk
+        # and opening a directory that merely looks like a source file, such as
+        # node_modules/popper.js (which would raise Errno::EISDIR).
+        if File.directory?(path)
+          next if /^\./ =~ file_name
+          next if IGNORED_DIRS.include?(file_name)
+
           stats.add(calculate_directory_statistics(path, pattern))
+          next
         end
 
         next unless file_name =~ pattern
diff --git a/test/fixtures/packwerk_app/app/controllers/core_controller.rb b/test/fixtures/packwerk_app/app/controllers/core_controller.rb
new file mode 100644
index 0000000..de30130
--- /dev/null
+++ b/test/fixtures/packwerk_app/app/controllers/core_controller.rb
@@ -0,0 +1,4 @@
+class CoreController
+  def index
+  end
+end
diff --git a/test/fixtures/packwerk_app/app/models/core_model.rb b/test/fixtures/packwerk_app/app/models/core_model.rb
new file mode 100644
index 0000000..304b2f4
--- /dev/null
+++ b/test/fixtures/packwerk_app/app/models/core_model.rb
@@ -0,0 +1,5 @@
+class CoreModel
+  def core_method
+    true
+  end
+end
diff --git a/test/fixtures/packwerk_app/components/billing/app/models/billing_model.rb b/test/fixtures/packwerk_app/components/billing/app/models/billing_model.rb
new file mode 100644
index 0000000..e770e2a
--- /dev/null
+++ b/test/fixtures/packwerk_app/components/billing/app/models/billing_model.rb
@@ -0,0 +1,4 @@
+class BillingModel
+  def charge
+  end
+end
diff --git a/test/fixtures/packwerk_app/components/billing/lib/billing.rb b/test/fixtures/packwerk_app/components/billing/lib/billing.rb
new file mode 100644
index 0000000..127724a
--- /dev/null
+++ b/test/fixtures/packwerk_app/components/billing/lib/billing.rb
@@ -0,0 +1,2 @@
+module Billing
+end
diff --git a/test/fixtures/packwerk_app/lib/core_lib.rb b/test/fixtures/packwerk_app/lib/core_lib.rb
new file mode 100644
index 0000000..7d434eb
--- /dev/null
+++ b/test/fixtures/packwerk_app/lib/core_lib.rb
@@ -0,0 +1,3 @@
+module CoreLib
+  CONST = 1
+end
diff --git a/test/fixtures/packwerk_app/package.yml b/test/fixtures/packwerk_app/package.yml
new file mode 100644
index 0000000..a04d956
--- /dev/null
+++ b/test/fixtures/packwerk_app/package.yml
@@ -0,0 +1 @@
+enforce_dependencies: true
diff --git a/test/fixtures/packwerk_app/packs/pack1/app/controllers/pack1_controller.rb b/test/fixtures/packwerk_app/packs/pack1/app/controllers/pack1_controller.rb
new file mode 100644
index 0000000..df67f90
--- /dev/null
+++ b/test/fixtures/packwerk_app/packs/pack1/app/controllers/pack1_controller.rb
@@ -0,0 +1,4 @@
+class Pack1Controller
+  def show
+  end
+end
diff --git a/test/fixtures/packwerk_app/packs/pack1/app/models/pack1_model.rb b/test/fixtures/packwerk_app/packs/pack1/app/models/pack1_model.rb
new file mode 100644
index 0000000..db04aaf
--- /dev/null
+++ b/test/fixtures/packwerk_app/packs/pack1/app/models/pack1_model.rb
@@ -0,0 +1,6 @@
+class Pack1Model
+  def m1
+  end
+  def m2
+  end
+end
diff --git a/test/fixtures/packwerk_app/packs/pack1/lib/pack1_lib.rb b/test/fixtures/packwerk_app/packs/pack1/lib/pack1_lib.rb
new file mode 100644
index 0000000..9a5ee10
--- /dev/null
+++ b/test/fixtures/packwerk_app/packs/pack1/lib/pack1_lib.rb
@@ -0,0 +1,2 @@
+module Pack1Lib
+end
diff --git a/test/fixtures/packwerk_app/packs/pack1/package.yml b/test/fixtures/packwerk_app/packs/pack1/package.yml
new file mode 100644
index 0000000..878f981
--- /dev/null
+++ b/test/fixtures/packwerk_app/packs/pack1/package.yml
@@ -0,0 +1,3 @@
+enforce_dependencies: true
+dependencies:
+  - packs/pack2
diff --git a/test/fixtures/packwerk_app/packs/pack1/spec/models/pack1_model_spec.rb b/test/fixtures/packwerk_app/packs/pack1/spec/models/pack1_model_spec.rb
new file mode 100644
index 0000000..61d1620
--- /dev/null
+++ b/test/fixtures/packwerk_app/packs/pack1/spec/models/pack1_model_spec.rb
@@ -0,0 +1,4 @@
+describe Pack1Model do
+  it "works" do
+  end
+end
diff --git a/test/fixtures/packwerk_app/packs/pack2/app/models/pack2_model.rb b/test/fixtures/packwerk_app/packs/pack2/app/models/pack2_model.rb
new file mode 100644
index 0000000..d20cb3c
--- /dev/null
+++ b/test/fixtures/packwerk_app/packs/pack2/app/models/pack2_model.rb
@@ -0,0 +1,4 @@
+class Pack2Model
+  def only
+  end
+end
diff --git a/test/fixtures/packwerk_app/packs/pack2/test/models/pack2_model_test.rb b/test/fixtures/packwerk_app/packs/pack2/test/models/pack2_model_test.rb
new file mode 100644
index 0000000..a097925
--- /dev/null
+++ b/test/fixtures/packwerk_app/packs/pack2/test/models/pack2_model_test.rb
@@ -0,0 +1,4 @@
+class Pack2ModelTest
+  def test_it
+  end
+end
diff --git a/test/lib/rails_stats/console_formatter_test.rb b/test/lib/rails_stats/console_formatter_test.rb
index 60f318a..e7f6e93 100644
--- a/test/lib/rails_stats/console_formatter_test.rb
+++ b/test/lib/rails_stats/console_formatter_test.rb
@@ -22,5 +22,27 @@
 
       assert_equal ['--format', 'text'], received_args
     end
+
+    it 'prints a section per pack plus a Core and a grand total for packwerk apps' do
+      root_directory = File.absolute_path('./test/fixtures/packwerk_app')
+      calculator = RailsStats::StatsCalculator.new(root_directory)
+      formatter  = RailsStats::ConsoleFormatter.new(calculator)
+
+      original = Bundler::Stats::CLI.method(:start)
+      Bundler::Stats::CLI.define_singleton_method(:start) { |args = []| nil }
+
+      begin
+        output, _ = capture_io { formatter.to_s }
+      ensure
+        Bundler::Stats::CLI.singleton_class.remove_method(:start)
+        Bundler::Stats::CLI.define_singleton_method(:start, original)
+      end
+
+      assert_includes output, '== Core Application =='
+      assert_includes output, '== Pack: packs/pack1 =='
+      assert_includes output, '== Pack: packs/pack2 =='
+      assert_includes output, '== Pack: components/billing =='
+      assert_includes output, '== Total (all packs) =='
+    end
   end
 end
diff --git a/test/lib/rails_stats/pack_finder_test.rb b/test/lib/rails_stats/pack_finder_test.rb
new file mode 100644
index 0000000..00a4e51
--- /dev/null
+++ b/test/lib/rails_stats/pack_finder_test.rb
@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+describe RailsStats::PackFinder do
+  describe ".find" do
+    it "detects packwerk packs and component-based components" do
+      root = File.absolute_path("./test/fixtures/packwerk_app")
+
+      packs = RailsStats::PackFinder.find(root).map { |path| path.sub("#{root}/", "") }
+
+      assert_equal ["components/billing", "packs/pack1", "packs/pack2"], packs.sort
+    end
+
+    it "does not treat the root package.yml as a pack" do
+      root = File.absolute_path("./test/fixtures/packwerk_app")
+
+      packs = RailsStats::PackFinder.find(root)
+
+      refute_includes packs, root
+    end
+
+    it "returns an empty list for a plain Rails app" do
+      root = File.absolute_path("./test/dummy")
+
+      assert_empty RailsStats::PackFinder.find(root)
+    end
+  end
+end
diff --git a/test/lib/rails_stats/stats_calculator_test.rb b/test/lib/rails_stats/stats_calculator_test.rb
new file mode 100644
index 0000000..f69c0f3
--- /dev/null
+++ b/test/lib/rails_stats/stats_calculator_test.rb
@@ -0,0 +1,64 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+describe RailsStats::StatsCalculator do
+  describe "#statistics_by_group" do
+    before do
+      root = File.absolute_path("./test/fixtures/packwerk_app")
+      @calculator = RailsStats::StatsCalculator.new(root)
+    end
+
+    it "reports that the app is split into packs" do
+      assert @calculator.packs?
+    end
+
+    it "groups statistics per pack/component with Core first" do
+      groups = @calculator.statistics_by_group.keys
+
+      assert_equal "Core", groups.first
+      assert_equal ["Core", "components/billing", "packs/pack1", "packs/pack2"], groups
+    end
+
+    it "attributes each pack's files to its own group" do
+      by_group = @calculator.statistics_by_group
+
+      assert_equal 1, by_group["packs/pack1"]["Models"].files_total
+      assert_equal 1, by_group["packs/pack1"]["Controllers"].files_total
+      assert_equal 1, by_group["packs/pack1"]["Model Tests"].files_total
+      assert_equal 1, by_group["packs/pack2"]["Models"].files_total
+      assert_equal 1, by_group["components/billing"]["Models"].files_total
+    end
+
+    it "keeps core files in the Core group" do
+      core = @calculator.statistics_by_group["Core"]
+
+      assert_equal 1, core["Models"].files_total
+      assert_equal 1, core["Controllers"].files_total
+      assert_equal 1, core["Libraries"].files_total
+    end
+
+    it "includes pack lib directories that are not gems" do
+      assert_equal 1, @calculator.statistics_by_group["packs/pack1"]["Libraries"].files_total
+    end
+
+    it "has a grand total that matches the sum of every group" do
+      _code, _tests, grand = @calculator.totals_for(@calculator.statistics)
+
+      per_group_files = @calculator.statistics_by_group.sum do |_group, stats|
+        _c, _t, group_grand = @calculator.totals_for(stats)
+        group_grand.files_total
+      end
+
+      assert_equal grand.files_total, per_group_files
+    end
+  end
+
+  describe "#packs?" do
+    it "is false for a plain Rails app" do
+      calculator = RailsStats::StatsCalculator.new(File.absolute_path("./test/dummy"))
+
+      refute calculator.packs?
+    end
+  end
+end
diff --git a/test/lib/rails_stats/util_test.rb b/test/lib/rails_stats/util_test.rb
new file mode 100644
index 0000000..dc52f47
--- /dev/null
+++ b/test/lib/rails_stats/util_test.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+require "test_helper"
+require "tmpdir"
+require "fileutils"
+
+describe RailsStats::Util do
+  describe ".calculate_directory_statistics" do
+    it "ignores vendored directories and never counts a directory that looks like a source file" do
+      Dir.mktmpdir do |dir|
+        # A real source file that should be counted.
+        File.write(File.join(dir, "real.rb"), "class Real\nend\n")
+
+        # A directory whose name matches the source-file pattern. It must be
+        # walked (its contents counted) but never opened as a file itself,
+        # which previously raised Errno::EISDIR.
+        weird = File.join(dir, "weird.rb")
+        FileUtils.mkdir_p(weird)
+        File.write(File.join(weird, "inner.rb"), "class Inner\nend\n")
+
+        # A vendored tree that must be skipped entirely, including a directory
+        # named like a JS file (e.g. node_modules/popper.js), the exact case
+        # that crashed on a packs app.
+        popper = File.join(dir, "node_modules", "popper.js")
+        FileUtils.mkdir_p(popper)
+        File.write(File.join(popper, "index.js"), "function noop() {}\n")
+
+        stats = RailsStats::Util.calculate_directory_statistics(dir)
+
+        # real.rb + weird.rb/inner.rb, nothing under node_modules.
+        assert_equal 2, stats.files_total
+      end
+    end
+
+    it "skips every ignored directory name" do
+      RailsStats::IGNORED_DIRS.each do |ignored|
+        Dir.mktmpdir do |dir|
+          vendored = File.join(dir, ignored)
+          FileUtils.mkdir_p(vendored)
+          File.write(File.join(vendored, "ignored.rb"), "class Ignored\nend\n")
+
+          stats = RailsStats::Util.calculate_directory_statistics(dir)
+
+          assert_equal 0, stats.files_total, "expected #{ignored}/ to be skipped"
+        end
+      end
+    end
+  end
+end