diff --git a/build_tests/simple/barrier.rb b/build_tests/simple/barrier.rb
new file mode 100644
index 0000000..2be4b6e
--- /dev/null
+++ b/build_tests/simple/barrier.rb
@@ -0,0 +1,24 @@
+class ThreadedTestBuilder < Rscons::Builder
+  def run(options)
+    if @command
+      puts "#{@target} finished"
+      true
+    else
+      @command = ["ruby", "-e", %[sleep #{@vars["delay"]}]]
+      register_command("ThreadedTestBuilder #{@target}", @command)
+    end
+  end
+end
+
+build do
+  Environment.new do |env|
+    env.add_builder(ThreadedTestBuilder)
+    env.ThreadedTestBuilder("T3", [], "delay" => 3)
+    env.ThreadedTestBuilder("T2", [], "delay" => 1.0)
+    env.ThreadedTestBuilder("T1", [], "delay" => 0.5)
+    env.barrier
+    env.ThreadedTestBuilder("T6", [], "delay" => 1.5)
+    env.ThreadedTestBuilder("T5", [], "delay" => 1.0)
+    env.ThreadedTestBuilder("T4", [], "delay" => 0.5)
+  end
+end
diff --git a/lib/rscons/environment.rb b/lib/rscons/environment.rb
index e6e439b..acf2a73 100644
--- a/lib/rscons/environment.rb
+++ b/lib/rscons/environment.rb
@@ -70,7 +70,7 @@ module Rscons
       @threads = {}
       @registered_build_dependencies = {}
       @side_effects = {}
-      @builder_set = BuilderSet.new(@registered_build_dependencies, @side_effects)
+      @builder_sets = []
       @build_targets = {}
       @user_deps = {}
       # Hash of builder name (String) => builder class (Class).
@@ -265,12 +265,16 @@ module Rscons
       @process_builder_waits = {}
       @process_builders_to_run = []
       begin
-        while @builder_set.size > 0 or @threads.size > 0 or @process_commands_waiting_to_run.size > 0
+        while @builder_sets.size > 0 or @threads.size > 0 or @process_commands_waiting_to_run.size > 0
           process_step
+          if @builder_sets.size > 0 and @builder_sets.first.empty? and @threads.empty? and @process_commands_waiting_to_run.empty? and @process_builders_to_run.empty?
+            # Remove empty BuilderSet when all other operations have completed.
+            @builder_sets.slice!(0)
+          end
           unless @process_failures.empty?
             # On a build failure, do not start any more builders or commands,
             # but let the threads that have already been started complete.
-            @builder_set.clear
+            @builder_sets.clear
             @process_commands_waiting_to_run.clear
           end
         end
@@ -290,7 +294,7 @@ module Rscons
     #
     # @return [void]
     def clear_targets
-      @builder_set.clear
+      @builder_sets.clear
     end
 
     # Define a build target.
@@ -319,7 +323,10 @@ module Rscons
           cache: Cache.instance,
           env: self,
           vars: vars)
-        @builder_set << builder
+        if @builder_sets.empty?
+          @builder_sets << build_builder_set
+        end
+        @builder_sets.last << builder
         @build_targets[target] = builder
         builder
       else
@@ -528,8 +535,26 @@ module Rscons
       @build_targets[target]
     end
 
+    # Mark a "barrier" point.
+    #
+    # Rscons will wait for all build targets registered before the barrier to
+    # be built before beginning to build any build targets registered after
+    # the barrier. In other words, Rscons will not parallelize build operations
+    # across a barrier.
+    def barrier
+      @builder_sets << build_builder_set
+    end
+
     private
 
+    # Build a BuilderSet.
+    #
+    # @return [BuilderSet]
+    #   New {BuilderSet} object.
+    def build_builder_set
+      BuilderSet.new(@registered_build_dependencies, @side_effects)
+    end
+
     # Run a builder and process its return value.
     #
     # @param builder [Builder]
@@ -639,7 +664,9 @@ module Rscons
       targets_still_building = @threads.reduce([]) do |result, (thread, obj)|
         result << builder_for_thread(thread).target
       end
-      builder = @builder_set.get_next_builder_to_run(targets_still_building)
+      if @builder_sets.size > 0
+        builder = @builder_sets[0].get_next_builder_to_run(targets_still_building)
+      end
 
       if builder
         builder.vars = @varset.merge(builder.vars)
diff --git a/spec/build_tests_spec.rb b/spec/build_tests_spec.rb
index aac7a34..b3617dc 100644
--- a/spec/build_tests_spec.rb
+++ b/spec/build_tests_spec.rb
@@ -265,6 +265,27 @@ EOF
     ]
   end
 
+  it "supports barriers and prevents parallelizing builders across them" do
+    test_dir "simple"
+    result = run_rscons(rsconscript: "barrier.rb", rscons_args: %w[-j 3])
+    expect(result.stderr).to eq ""
+    slines = lines(result.stdout).select {|line| line =~ /T\d/}
+    expect(slines).to eq [
+      "ThreadedTestBuilder T3",
+      "ThreadedTestBuilder T2",
+      "ThreadedTestBuilder T1",
+      "T1 finished",
+      "T2 finished",
+      "T3 finished",
+      "ThreadedTestBuilder T6",
+      "ThreadedTestBuilder T5",
+      "ThreadedTestBuilder T4",
+      "T4 finished",
+      "T5 finished",
+      "T6 finished",
+    ]
+  end
+
   it "expands target and source paths starting with ^/ to be relative to the build root" do
     test_dir("typical")
     result = run_rscons(rsconscript: "carat.rb")