diff --git a/lib/overcommit/hook_context/pre_commit.rb b/lib/overcommit/hook_context/pre_commit.rb
index 289eaffe..c7c82ec7 100644
--- a/lib/overcommit/hook_context/pre_commit.rb
+++ b/lib/overcommit/hook_context/pre_commit.rb
@@ -7,7 +7,30 @@ module Overcommit::HookContext
   # This includes staged files, which lines of those files have been modified,
   # etc. It is also responsible for saving/restoring the state of the repo so
   # hooks only inspect staged changes.
-  class PreCommit < Base
+  class PreCommit < Base # rubocop:disable ClassLength
+    # Returns whether this hook run was triggered by `git commit --amend`
+    def amendment?
+      return @amendment unless @amendment.nil?
+
+      cmd = Overcommit::Utils.parent_command
+      amend_pattern = 'commit(\s.*)?\s--amend(\s|$)'
+
+      return @amendment if
+        # True if the command is a commit with the --amend flag
+        @amendment = !(/\s#{amend_pattern}/ =~ cmd).nil?
+
+      # Check for git aliases that call `commit --amend`
+      `git config --get-regexp '^alias\\.' '#{amend_pattern}'`.
+        scan(/alias\.([-\w]+)/). # Extract the alias
+        each do |match|
+          return @amendment if
+            # True if the command uses a git alias for `commit --amend`
+            @amendment = !(/git\s+#{match[0]}/ =~ cmd).nil?
+        end
+
+      @amendment
+    end
+
     # Stash unstaged contents of files so hooks don't see changes that aren't
     # about to be committed.
     def setup_environment
@@ -64,7 +87,16 @@ def cleanup_environment
     # Get a list of added, copied, or modified files that have been staged.
     # Renames and deletions are ignored, since there should be nothing to check.
     def modified_files
-      @modified_files ||= Overcommit::GitRepo.modified_files(staged: true)
+      unless @modified_files
+        @modified_files = Overcommit::GitRepo.modified_files(staged: true)
+
+        # Include files modified in last commit if amending
+        if amendment?
+          subcmd = 'show --format=%n'
+          @modified_files += Overcommit::GitRepo.modified_files(subcmd: subcmd)
+        end
+      end
+      @modified_files
     end
 
     # @deprecated
@@ -78,8 +110,18 @@ def modified_lines(file)
     # changed in a specified file.
     def modified_lines_in_file(file)
       @modified_lines ||= {}
-      @modified_lines[file] ||=
-        Overcommit::GitRepo.extract_modified_lines(file, staged: true)
+      unless @modified_lines[file]
+        @modified_lines[file] =
+          Overcommit::GitRepo.extract_modified_lines(file, staged: true)
+
+        # Include lines modified in last commit if amending
+        if amendment?
+          subcmd = 'show --format=%n'
+          @modified_lines[file] +=
+            Overcommit::GitRepo.extract_modified_lines(file, subcmd: subcmd)
+        end
+      end
+      @modified_lines[file]
     end
 
     private
diff --git a/lib/overcommit/utils.rb b/lib/overcommit/utils.rb
index 540937f3..20bcfaf4 100644
--- a/lib/overcommit/utils.rb
+++ b/lib/overcommit/utils.rb
@@ -101,6 +101,11 @@ def in_path?(cmd)
         false
       end
 
+      # Return the parent command that triggered this hook run
+      def parent_command
+        `ps -ocommand= -p #{Process.ppid}`.chomp
+      end
+
       # Execute a command in a subprocess, capturing exit status and output from
       # both standard and error streams.
       #
diff --git a/spec/overcommit/hook_context/pre_commit_spec.rb b/spec/overcommit/hook_context/pre_commit_spec.rb
index b1af2d0e..0d4cd8fb 100644
--- a/spec/overcommit/hook_context/pre_commit_spec.rb
+++ b/spec/overcommit/hook_context/pre_commit_spec.rb
@@ -7,6 +7,63 @@
   let(:input) { double('input') }
   let(:context) { described_class.new(config, args, input) }
 
+  describe '#amendment?' do
+    subject { context.amendment? }
+
+    before do
+      Overcommit::Utils.stub(:parent_command).and_return(command)
+    end
+
+    context 'when amending a commit using `git commit --amend`' do
+      let(:command) { 'git commit --amend' }
+
+      it { should == true }
+    end
+
+    context 'when amending a commit using a git alias' do
+      around do |example|
+        repo do
+          `git config alias.amend 'commit --amend'`
+          `git config alias.other-amend 'commit --amend'`
+          example.run
+        end
+      end
+
+      context 'when using one of multiple aliases' do
+        let(:command) { 'git amend' }
+
+        it { should == true }
+      end
+
+      context 'when using another of multiple aliases' do
+        let(:command) { 'git other-amend' }
+
+        it { should == true }
+      end
+    end
+
+    context 'when not amending a commit' do
+      context 'using `git commit`' do
+        let(:command) { 'git commit' }
+
+        it { should == false }
+      end
+
+      context 'using a git alias containing "--amend"' do
+        let(:command) { 'git no--amend' }
+
+        around do |example|
+          repo do
+            `git config alias.no--amend commit`
+            example.run
+          end
+        end
+
+        it { should == false }
+      end
+    end
+  end
+
   describe '#setup_environment' do
     subject { context.setup_environment }
 
@@ -261,6 +318,10 @@
   describe '#modified_files' do
     subject { context.modified_files }
 
+    before do
+      context.stub(:amendment?).and_return(false)
+    end
+
     it 'does not include submodules' do
       submodule = repo do
         `touch foo`
@@ -324,5 +385,80 @@
 
       it { should be_empty }
     end
+
+    context 'when amending last commit' do
+      around do |example|
+        repo do
+          FileUtils.touch('some-file')
+          `git add some-file`
+          `git commit -m 'Initial commit'`
+          FileUtils.touch('other-file')
+          `git add other-file`
+          example.run
+        end
+      end
+
+      before do
+        context.stub(:amendment?).and_return(true)
+      end
+
+      it { should =~ [File.expand_path('some-file'), File.expand_path('other-file')] }
+    end
+  end
+
+  describe '#modified_lines_in_file' do
+    let(:modified_file) { 'some-file' }
+    subject { context.modified_lines_in_file(modified_file) }
+
+    before do
+      context.stub(:amendment?).and_return(false)
+    end
+
+    context 'when file contains a trailing newline' do
+      around do |example|
+        repo do
+          File.open(modified_file, 'w') { |f| (1..3).each { |i| f.write("#{i}\n") } }
+          `git add #{modified_file}`
+          example.run
+        end
+      end
+
+      it { should == Set.new(1..3) }
+    end
+
+    context 'when file does not contain a trailing newline' do
+      around do |example|
+        repo do
+          File.open(modified_file, 'w') do |f|
+            (1..2).each { |i| f.write("#{i}\n") }
+            f.write(3)
+          end
+
+          `git add #{modified_file}`
+          example.run
+        end
+      end
+
+      it { should == Set.new(1..3) }
+    end
+
+    context 'when amending last commit' do
+      around do |example|
+        repo do
+          File.open(modified_file, 'w') { |f| (1..3).each { |i| f.write("#{i}\n") } }
+          `git add #{modified_file}`
+          `git commit -m "Add files"`
+          File.open(modified_file, 'a') { |f| f.puts 4 }
+          `git add #{modified_file}`
+          example.run
+        end
+      end
+
+      before do
+        context.stub(:amendment?).and_return(true)
+      end
+
+      it { should == Set.new(1..4) }
+    end
   end
 end
diff --git a/spec/overcommit/utils_spec.rb b/spec/overcommit/utils_spec.rb
index 91bb32ef..215a1aee 100644
--- a/spec/overcommit/utils_spec.rb
+++ b/spec/overcommit/utils_spec.rb
@@ -121,6 +121,16 @@
     # rubocop:enable Metrics/LineLength
   end
 
+  describe '.parent_command' do
+    subject { described_class.parent_command }
+
+    before do
+      Process.stub(:ppid) { Process.pid }
+    end
+
+    it { should =~ /rspec/ }
+  end
+
   describe '.execute' do
     let(:arguments) { %w[echo -n Hello World] }
     subject { described_class.execute(arguments) }