diff --git a/semver.lua b/semver.lua
index 998de58..4c03a55 100644
--- a/semver.lua
+++ b/semver.lua
@@ -16,12 +16,13 @@ local function present(value)
   return value and value ~= ''
 end
 
-local function parsePrerelease(extra)
+local function parseExtra(extra)
   if not present(extra) then return end
 
-  local prerelease = extra:match("-([%w-][%.%w-]*)")
-  assert(prerelease, ("The prerelease %q must start with a dash and be followed by dashes, alphanumerics or dots."):format(extra))
-  return prerelease
+  local sign, data = extra:match("([-+])([%w-][%.%w-]*)")
+  assert(sign and data, ("The string %q must start with - or + followed by dashes, alphanumerics or dots."):format(extra))
+  if sign == "-" then return data, nil end
+  return nil,data
 end
 
 -- return 0 if a == b, -1 if a < b, and 1 if a > b
@@ -88,14 +89,17 @@ function mt:__eq(other)
   return self.major == other.major and
          self.minor == other.minor and
          self.patch == other.patch and
-         self.prerelease == other.prerelease
+         self.prerelease == other.prerelease and
+         self.build == other.build
 end
 function mt:__lt(other)
   return self.major < other.major or
          self.minor < other.minor or
          self.patch < other.patch or
          (self.prerelease and not other.prerelease) or
-         smallerExtra(self.prerelease, other.prerelease)
+         smallerExtra(self.prerelease, other.prerelease) or
+         (not self.build and other.build) or
+         smallerExtra(self.build, other.build)
 end
 function mt:__pow(other)
   return self.major == other.major and
@@ -103,7 +107,9 @@ function mt:__pow(other)
 end
 function mt:__tostring()
   local buffer = { ("%d.%d.%d"):format(self.major, self.minor, self.patch) }
-  if self.prerelease then table.insert(buffer, "-" .. self.prerelease) end
+  if self.prerelease then buffer[2] = "-" .. self.prerelease
+  elseif self.build  then buffer[2] = "+" .. self.build
+  end
   return table.concat(buffer)
 end
 
@@ -126,9 +132,9 @@ version = function(major, minor, patch, extra)
   checkPositiveInteger(minor, "minor")
   checkPositiveInteger(patch, "patch")
 
-  local prerelease = parsePrerelease(extra)
+  local prerelease, build = parseExtra(extra)
 
-  local result = {major=major, minor=minor, patch=patch, prerelease=prerelease}
+  local result = {major=major, minor=minor, patch=patch, prerelease=prerelease, build=build}
   return setmetatable(result, mt)
 end
 
diff --git a/spec/semver_spec.lua b/spec/semver_spec.lua
index a6570ba..2e81d28 100644
--- a/spec/semver_spec.lua
+++ b/spec/semver_spec.lua
@@ -1,10 +1,11 @@
 local v = require 'semver'
 
-local function checkVersion(ver, major, minor, patch, prerelease)
+local function checkVersion(ver, major, minor, patch, prerelease, build)
   assert_equal(major, ver.major)
   assert_equal(minor, ver.minor)
   assert_equal(patch, ver.patch)
   assert_equal(prerelease, ver.prerelease)
+  assert_equal(build, ver.build)
 end
 
 context('semver', function()
@@ -24,9 +25,12 @@ context('semver', function()
         checkVersion(v(1), 1,0,0)
       end)
 
-      it('parses prereleases, if they exist', function()
+      it('parses prereleases', function()
         checkVersion(v(1,2,3,"-alpha"), 1,2,3,"alpha")
       end)
+      it('parses builds', function()
+        checkVersion(v(1,2,3,"+build.1"), 1,2,3,nil,"build.1")
+      end)
     end)
 
     describe("from strings", function()
@@ -45,6 +49,9 @@ context('semver', function()
       test("1.2.3-alpha", function()
         checkVersion( v'1.2.3-alpha', 1,2,3,'alpha' )
       end)
+      test("1.2.3+build.15", function()
+        checkVersion( v'1.2.3+build.15', 1,2,3,nil,'build.15' )
+      end)
     end)
 
     describe('errors', function()
@@ -84,6 +91,9 @@ context('semver', function()
     it("works with a prerelease", function()
       assert_equal("1.2.3-beta", tostring(v(1,2,3,'-beta')))
     end)
+    it("works with a build", function()
+      assert_equal("1.2.3+foobar", tostring(v(1,2,3, '+foobar')))
+    end)
   end)
 
   describe("==", function()
@@ -96,6 +106,9 @@ context('semver', function()
     it("false if all is the same except the prerelease", function()
       assert_not_equal(v(1,2,3), v'1.2.3-alpha')
     end)
+    it("false if all is the same except the build", function()
+      assert_not_equal(v(1,2,3), v'1.2.3+peter.1')
+    end)
   end)
 
   describe("<", function()
@@ -143,6 +156,31 @@ context('semver', function()
         assert_less_than(v'1.0.0-beta.11', v'1.0.0-rc.1')
       end)
     end)
+    describe("builds", function()
+      test("false if exact same build", function()
+        assert_false(v'1.0.0+build1' < v'1.0.0+build1')
+      end)
+      test("a regular (not-build) version is always less than a build version", function()
+        assert_less_than(v'1.0.0', v'1.0.0+12')
+      end)
+      test("identifiers with only digits are compared numerically", function()
+        assert_less_than(v'1.0.0+1', v'1.0.0+2')
+        assert_less_than(v'1.0.0+2', v'1.0.0+10')
+      end)
+      test("idendifiers with letters or dashes are compared lexiconumerically", function()
+        assert_less_than(v'1.0.0+build1',  v'1.0.0+build2')
+        assert_less_than(v'1.0.0+build10', v'1.0.0+build2')
+      end)
+      test("numerical ids always have less priority than lexiconumericals", function()
+        assert_less_than(v'1.0.0+1', v'1.0.0+build1')
+        assert_less_than(v'1.0.0+2', v'1.0.0+1build')
+      end)
+      test("identifiers can be separated by colons; they must be compared individually", function()
+        assert_less_than(v'1.0.0+0.3.7', v'1.3.7+build')
+        assert_less_than(v'1.3.7+build', v'1.3.7+build.2.b8f12d7')
+        assert_less_than(v'1.3.7+build.2.b8f12d7', v'1.3.7+build.11.e0f985a')
+      end)
+    end)
   end)
 
   describe("nextPatch", function()