diff --git a/lib/strong_ruby/runner.rb b/lib/strong_ruby/runner.rb
index 10dd83e..afd4a93 100644
--- a/lib/strong_ruby/runner.rb
+++ b/lib/strong_ruby/runner.rb
@@ -1,7 +1,120 @@
+require 'optparse'
+require 'find'
+
+require 'strong_ruby/version'
+
 module StrongRuby
   # Parent class for runner tools.
   class Runner
     def self.run(options)
+      new.execute(options)
+    end
+
+    attr_reader :errors, :warnings, :aborting
+
+    def initialize
+      @option_parser = OptionParser.new { |opts| setup_option_parsing(opts) }
+      @files         = []
+      @errors        = []
+      @warnings      = []
+      @list_files    = false
+      @aborting      = false
+    end
+
+    def execute(options)
+      parse_options(options)
+      prepare_checker
+
+      process_files
+    end
+
+    private
+
+    def runner_name
+      raise NotImplementedError, "implement #{self.class}##{__callee__}"
+    end
+
+    # rubocop:disable MethodLength
+    def setup_option_parsing(opts)
+      opts.banner = "Usage: #{runner_name} [options] FILE|DIRECTORY..."
+
+      opts.on_tail '-h', '--help', 'Display this help message and exit' do
+        puts opts.help
+        puts <<-HELP
+
+  If you specify a DIRECTORY, then all *.srb files are fetched
+  from it recursively and appended to the file list.
+        HELP
+        exit
+      end
+
+      opts.on_tail '-V', '--version', 'Output version information and exit' do
+        puts "#{runner_name} #{VERSION}"
+        exit
+      end
+
+      opts.on_tail '-L', '--list-files', 'List target files' do
+        @list_files = true
+      end
+    end
+
+    def parse_options(options)
+      @option_parser.parse!(options)
+
+      # The rest of the method performs file retrieval. TODO RuboCop-ify.
+      options << Dir.pwd if options.empty?
+
+      options.each do |file_or_dir|
+        if File.directory?(file_or_dir)
+          Find.find(file_or_dir) do |path|
+            @files << path if path.end_with? '.srb'
+          end
+        else
+          @files << file_or_dir
+        end
+      end
+
+      return unless @files.empty?
+
+      $stderr.puts 'No StrongRuby files found'
+      exit 1
+    end
+    # rubocop:enable MethodLength
+
+    def prepare_checker
+      # TODO
+    end
+
+    def process_files
+      if @list_files
+        list_files(@files)
+      else
+        inspect_files(@files)
+      end
+    end
+
+    def list_files(paths)
+      paths.each do |path|
+        # NOTE Inlined from Rubocop::PathUtil.relative_path
+        base_dir = Dir.pwd
+        if path.start_with?(base_dir)
+          path_name = path[(base_dir.length + 1)..-1]
+        else
+          path_name = Pathname.new(File.expand_path(path))
+          path_name.relative_path_from(Pathname.new(base_dir)).to_s
+        end
+        puts path_name
+      end
+    end
+
+    def inspect_files(paths)
+      paths.each do |_path|
+        # TODO
+      end
+    end
+
+    def process(_buffer)
+      raise NotImplementedError, "implement #{self.class}##{__callee__}"
     end
   end
 end
diff --git a/lib/strong_ruby/runner/strong_ruby.rb b/lib/strong_ruby/runner/strong_ruby.rb
index 7eaba43..66f1159 100644
--- a/lib/strong_ruby/runner/strong_ruby.rb
+++ b/lib/strong_ruby/runner/strong_ruby.rb
@@ -1,6 +1,24 @@
 require 'strong_ruby/runner'
 
 module StrongRuby
+  # StrongRuby gradual typechecker.
   class Runner::StrongRuby < StrongRuby::Runner
+    def initialize
+      super
+    end
+
+    private
+
+    def runner_name
+      'strongruby'
+    end
+
+    def setup_option_parsing(opts)
+      super
+    end
+
+    def process(buffer)
+      # TODO
+    end
   end
 end
diff --git a/lib/strong_ruby/runner/strong_ruby_c.rb b/lib/strong_ruby/runner/strong_ruby_c.rb
index 3e889cf..7814c10 100644
--- a/lib/strong_ruby/runner/strong_ruby_c.rb
+++ b/lib/strong_ruby/runner/strong_ruby_c.rb
@@ -1,6 +1,24 @@
 require 'strong_ruby/runner'
 
 module StrongRuby
+  # StrongRuby to Ruby transpiler.
   class Runner::StrongRubyC < StrongRuby::Runner
+    def initialize
+      super
+    end
+
+    private
+
+    def runner_name
+      'strongrubyc'
+    end
+
+    def setup_option_parsing(opts)
+      super
+    end
+
+    def process(buffer)
+      # TODO
+    end
   end
 end
diff --git a/strongruby.gemspec b/strongruby.gemspec
index dd7f424..5d9587a 100644
--- a/strongruby.gemspec
+++ b/strongruby.gemspec
@@ -16,4 +16,6 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
   spec.test_files    = spec.files.grep(%r{^test/})
   spec.require_paths = ['lib']
+
+  spec.add_runtime_dependency 'rubocop', '~> 0.40'
 end