diff --git a/doc-src/SASS_CHANGELOG.md b/doc-src/SASS_CHANGELOG.md
index 384fb47f..bc55f7eb 100644
--- a/doc-src/SASS_CHANGELOG.md
+++ b/doc-src/SASS_CHANGELOG.md
@@ -102,7 +102,7 @@ and all of them will be watched:
 File and directory watching is accessible from Ruby,
 using the {Sass::Plugin#watch} function.
 
-### Bulk Updating
+#### Bulk Updating
 
 Another new flag for the `sass` command-line utility is `--update`.
 It checks a group of Sass files to see if their CSS needs to be updated,
@@ -118,14 +118,31 @@ In fact, `--update` work exactly the same as `--watch`,
 except that it doesn't continue watching the files
 after the first check.
 
-### Variable Names
+### Syntax
 
-SassScript variable names may now contain hyphens.
+#### Variable and Mixin Names
+
+SassScript variable and mixin names may now contain hyphens.
+In fact, they may be any valid CSS3 identifier.
 For example:
 
     !prettiest-color = #542FA9
+    =pretty-text
+      color = !prettiest-color
 
-### Single-Quoted Strings
+In order to allow frameworks like [Compass](http://compass-style.org)
+to use hyphens in variable names
+while maintaining backwards-compatibility,
+variables and mixins using hyphens may be referred to
+with underscores, and vice versa.
+For example:
+
+    !prettiest-color = #542FA9
+    .pretty
+      // Using an underscore instead of a hyphen works
+      color = !prettiest_color
+
+#### Single-Quoted Strings
 
 SassScript now supports single-quoted strings.
 They behave identically to double-quoted strings,
diff --git a/extra/haml-mode.el b/extra/haml-mode.el
index 32cb84ca..d5a42bd1 100644
--- a/extra/haml-mode.el
+++ b/extra/haml-mode.el
@@ -30,6 +30,7 @@
 (require 'textile-mode nil t)
 (require 'markdown-mode nil t)
 (require 'javascript-mode "javascript" t)
+(require 'js nil t)
 
 
 ;; User definable variables
@@ -172,13 +173,15 @@ This requires that `css-mode' is available.
 (defun haml-highlight-js-filter-block (limit)
   "If a :javascript filter is found within LIMIT, highlight it.
 
-This requires that Karl Landström's \"javascript.el\" be available."
-  (if (boundp 'js-font-lock-keywords-3)
-      (haml-fontify-filter-region "javascript"
-                                  limit
-                                  js-font-lock-keywords-3
-                                  javascript-mode-syntax-table
-                                  nil)))
+This requires that Karl Landström's javascript mode be available, either as the
+\"js.el\" bundled with Emacs 23, or as \"javascript.el\" found in ELPA and
+elsewhere."
+  (let ((keywords (or (and (featurep 'js) js--font-lock-keywords-3)
+                      (and (featurep 'javascript-mode) js-font-lock-keywords-3)))
+        (syntax-table (or (and (featurep 'js) js-mode-syntax-table)
+                          (and (featurep 'javascript-mode) javascript-mode-syntax-table))))
+    (when keywords
+      (haml-fontify-filter-region "javascript" limit keywords syntax-table nil))))
 
 (defun haml-highlight-textile-filter-block (limit)
   "If a :textile filter is found within LIMIT, highlight it.
diff --git a/lib/sass/engine.rb b/lib/sass/engine.rb
index be524895..acf7a51d 100644
--- a/lib/sass/engine.rb
+++ b/lib/sass/engine.rb
@@ -494,8 +494,10 @@ WARNING
       nil
     end
 
+    # @private
+    MIXIN_DEF_RE = /^=\s*(#{Sass::SCSS::RX::IDENT})(.*)$/
     def parse_mixin_definition(line)
-      name, arg_string = line.text.scan(/^=\s*([^(]+)(.*)$/).first
+      name, arg_string = line.text.scan(MIXIN_DEF_RE).first
       raise SyntaxError.new("Invalid mixin \"#{line.text[1..-1]}\".") if name.nil?
 
       offset = line.offset + line.text.size - arg_string.size
@@ -505,8 +507,10 @@ WARNING
       Tree::MixinDefNode.new(name, args)
     end
 
+    # @private
+    MIXIN_INCLUDE_RE = /^\+\s*(#{Sass::SCSS::RX::IDENT})(.*)$/
     def parse_mixin_include(line, root)
-      name, arg_string = line.text.scan(/^\+\s*([^(]+)(.*)$/).first
+      name, arg_string = line.text.scan(MIXIN_INCLUDE_RE).first
       raise SyntaxError.new("Invalid mixin include \"#{line.text}\".") if name.nil?
 
       offset = line.offset + line.text.size - arg_string.size
diff --git a/lib/sass/environment.rb b/lib/sass/environment.rb
index 5440c213..4f0c577c 100644
--- a/lib/sass/environment.rb
+++ b/lib/sass/environment.rb
@@ -43,10 +43,16 @@ module Sass
       def inherited_hash(name)
         class_eval <<RUBY, __FILE__, __LINE__ + 1
           def #{name}(name)
-            @#{name}s[name] || @parent && @parent.#{name}(name)
+            _#{name}(name.gsub('_', '-'))
           end
 
+          def _#{name}(name)
+            @#{name}s[name] || @parent && @parent._#{name}(name)
+          end
+          protected :_#{name}
+
           def set_#{name}(name, value)
+            name = name.gsub('_', '-')
             @#{name}s[name] = value unless try_set_#{name}(name, value)
           end
 
@@ -63,7 +69,7 @@ module Sass
           protected :try_set_#{name}
 
           def set_local_#{name}(name, value)
-            @#{name}s[name] = value
+            @#{name}s[name.gsub('_', '-')] = value
           end
 RUBY
       end
diff --git a/lib/sass/script.rb b/lib/sass/script.rb
index 56b0988a..c2a32cb9 100644
--- a/lib/sass/script.rb
+++ b/lib/sass/script.rb
@@ -18,11 +18,11 @@ module Sass
 
     # The regular expression used to parse variables.
     # @private
-    MATCH = /^!([a-zA-Z_]\w*)\s*((?:\|\|)?=)\s*(.+)/
+    MATCH = /^!(#{Sass::SCSS::RX::IDENT})\s*((?:\|\|)?=)\s*(.+)/
 
     # The regular expression used to validate variables without matching.
     # @private
-    VALIDATE = /^![a-zA-Z_]\w*$/
+    VALIDATE = /^!#{Sass::SCSS::RX::IDENT}$/
 
     # Parses a string of SassScript
     #
diff --git a/test/sass/engine_test.rb b/test/sass/engine_test.rb
index b592fc44..190fa0ee 100755
--- a/test/sass/engine_test.rb
+++ b/test/sass/engine_test.rb
@@ -63,6 +63,8 @@ MSG
     "foo\n  @import #{File.dirname(__FILE__)}/templates/basic" => "Import directives may only be used at the root of a document.",
     %Q{!foo = "bar" "baz" !} => %Q{Syntax error in '"bar" "baz" !' at character 20.},
     "=foo\n  :color red\n.bar\n  +bang" => "Undefined mixin 'bang'.",
+    "=foo\n  :color red\n.bar\n  +bang_bop" => "Undefined mixin 'bang_bop'.",
+    "=foo\n  :color red\n.bar\n  +bang-bop" => "Undefined mixin 'bang-bop'.",
     ".bar\n  =foo\n    :color red\n" => ["Mixins may only be defined at the root of a document.", 2],
     "=foo\n  :color red\n.bar\n  +foo\n    :color red" => "Illegal nesting: Nothing may be nested beneath mixin directives.",
     "    a\n  b: c" => ["Indenting at the beginning of the document is illegal.", 1],
@@ -90,6 +92,8 @@ MSG
     "@if false\n@else if " => "Invalid else directive '@else if': expected 'if <expr>'.",
     "a\n  !b = 12\nc\n  d = !b" => 'Undefined variable: "!b".',
     "=foo\n  !b = 12\nc\n  +foo\n  d = !b" => 'Undefined variable: "!b".',
+    "c\n  d = !b-foo" => 'Undefined variable: "!b-foo".',
+    "c\n  d = !b_foo" => 'Undefined variable: "!b_foo".',
     '@for !a from "foo" to 1' => '"foo" is not an integer.',
     '@for !a from 1 to "2"' => '"2" is not an integer.',
     '@for !a from 1 to "foo"' => '"foo" is not an integer.',
@@ -738,6 +742,37 @@ three
 SASS
   end
 
+  def test_hyphen_underscore_insensitive_mixins
+    assert_equal(<<CSS, render(<<SASS))
+a {
+  b: 12;
+  c: foo; }
+CSS
+=mixin-hyphen
+  b: 12
+
+=mixin_under
+  c: foo
+
+a
+  +mixin_hyphen
+  +mixin-under
+SASS
+  end
+
+  def test_css_identifier_mixin
+    assert_equal(<<CSS, render(<<SASS))
+a {
+  foo: 12; }
+CSS
+=\\{foo\\(12\\)(!a)
+  foo = !a
+
+a
+  +\\{foo\\(12\\)(12)
+SASS
+  end
+
   def test_interpolation
     assert_equal("a-1 {\n  b-2-3: c-3; }\n", render(<<SASS))
 !a = 1
@@ -900,6 +935,41 @@ b
 SASS
   end
 
+  def test_hyphen_underscore_insensitive_variables
+    assert_equal(<<CSS, render(<<SASS))
+a {
+  b: c; }
+
+d {
+  e: 13;
+  f: foobar; }
+CSS
+!var-hyphen = 12
+!var_under = "foo"
+
+a
+  !var_hyphen = 1 + !var_hyphen
+  !var-under = !var-under + "bar"
+  b: c
+
+d
+  e = !var-hyphen
+  f = !var_under
+SASS
+  end
+
+  def test_css_identifier_variable
+    assert_equal(<<CSS, render(<<SASS))
+a {
+  b: 12; }
+CSS
+!\\{foo\\(12\\) = 12
+
+a
+  b = !\\{foo\\(12\\)
+SASS
+  end
+
   def test_argument_error
     assert_raise(Sass::SyntaxError) { render("a\n  b = hsl(1)") }
   end