diff --git a/README.md b/README.md
index 74b893001..03a1fda1b 100644
--- a/README.md
+++ b/README.md
@@ -67,6 +67,7 @@ Collection classes that were originally part of the (deprecated) `thread_safe` g
 
 * [Array](http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/Array.html) A thread-safe subclass of Ruby's standard [Array](http://ruby-doc.org/core-2.2.0/Array.html).
 * [Hash](http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/Hash.html) A thread-safe subclass of Ruby's standard [Hash](http://ruby-doc.org/core-2.2.0/Hash.html).
+* [Set](http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/Set.html) A thread-safe subclass of Ruby's standard [Set](http://ruby-doc.org/stdlib-2.4.0/libdoc/set/rdoc/Set.html).
 * [Map](http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/Map.html) A hash-like object that should have much better performance characteristics, especially under high concurrency, than `Concurrent::Hash`.
 * [Tuple](http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/Tuple.html) A fixed size array with volatile (synchronized, thread safe) getters/setters.
 
diff --git a/lib/concurrent.rb b/lib/concurrent.rb
index 1ea1f154e..d7377b54d 100644
--- a/lib/concurrent.rb
+++ b/lib/concurrent.rb
@@ -12,6 +12,7 @@
 require 'concurrent/atom'
 require 'concurrent/array'
 require 'concurrent/hash'
+require 'concurrent/set'
 require 'concurrent/map'
 require 'concurrent/tuple'
 require 'concurrent/async'
@@ -126,5 +127,4 @@
 # * Exclude features that don't make sense in Ruby
 # * Be small, lean, and loosely coupled
 module Concurrent
-
-end
+end
\ No newline at end of file
diff --git a/lib/concurrent/set.rb b/lib/concurrent/set.rb
new file mode 100644
index 000000000..ea6e85f09
--- /dev/null
+++ b/lib/concurrent/set.rb
@@ -0,0 +1,47 @@
+require 'concurrent/utility/engine'
+require 'concurrent/thread_safe/util'
+require 'set'
+
+module Concurrent
+  if Concurrent.on_cruby?
+
+    # Because MRI never runs code in parallel, the existing
+    # non-thread-safe structures should usually work fine.
+
+    # @!macro [attach] concurrent_Set
+    #
+    #   A thread-safe subclass of Set. This version locks against the object
+    #   itself for every method call, ensuring only one thread can be reading
+    #   or writing at a time. This includes iteration methods like `#each`.
+    #
+    #   @note `a += b` is **not** a **thread-safe** operation on
+    #   `Concurrent::Set`. It reads Set `a`, then it creates new `Concurrent::Set`
+    #   which is union of `a` and `b`, then it writes the union to `a`.
+    #   The read and write are independent operations they do not form a single atomic
+    #   operation therefore when two `+=` operations are executed concurrently updates
+    #   may be lost. Use `#merge` instead.
+    #
+    #   @see http://ruby-doc.org/stdlib-2.4.0/libdoc/set/rdoc/Set.html Ruby standard library `Set`
+    class Set < ::Set;
+    end
+
+  elsif Concurrent.on_jruby?
+    require 'jruby/synchronized'
+
+    # @!macro concurrent_Set
+    class Set < ::Set
+      include JRuby::Synchronized
+    end
+
+  elsif Concurrent.on_rbx? || Concurrent.on_truffle?
+    require 'monitor'
+    require 'concurrent/thread_safe/util/array_hash_rbx'
+
+    # @!macro concurrent_Set
+    class Set < ::Set
+    end
+
+    ThreadSafe::Util.make_synchronized_on_rbx Set
+  end
+end
+
diff --git a/spec/concurrent/array_spec.rb b/spec/concurrent/array_spec.rb
index 6f20c9d8c..5d6c7f72a 100644
--- a/spec/concurrent/array_spec.rb
+++ b/spec/concurrent/array_spec.rb
@@ -13,6 +13,7 @@ module Concurrent
           end
         end
       end.map(&:join)
+      expect(ary).to be_empty
     end
 
     describe '#slice' do
diff --git a/spec/concurrent/channel_spec.rb b/spec/concurrent/channel_spec.rb
index b0bce1346..339f98588 100644
--- a/spec/concurrent/channel_spec.rb
+++ b/spec/concurrent/channel_spec.rb
@@ -611,7 +611,7 @@ module Concurrent
           latch.wait(10)
           expect(actual).to eq expected
         end
-      end
+      end 
 
       context '.go_loop_via' do
 
diff --git a/spec/concurrent/hash_spec.rb b/spec/concurrent/hash_spec.rb
index 2e5e1061b..459e0a1ad 100644
--- a/spec/concurrent/hash_spec.rb
+++ b/spec/concurrent/hash_spec.rb
@@ -7,11 +7,12 @@ module Concurrent
         Thread.new do
           1000.times do |j|
             hsh[i * 1000 + j] = i
-            hsh[i * 1000 + j]
-            hsh.delete(i * 1000 + j)
+            expect(hsh[i * 1000 + j]).to eq(i)
+            expect(hsh.delete(i * 1000 + j)).to eq(i)
           end
         end
       end.map(&:join)
+      expect(hsh).to be_empty
     end
   end
 end
diff --git a/spec/concurrent/set_spec.rb b/spec/concurrent/set_spec.rb
new file mode 100644
index 000000000..6fe8766b1
--- /dev/null
+++ b/spec/concurrent/set_spec.rb
@@ -0,0 +1,20 @@
+require 'set'
+module Concurrent
+  RSpec.describe Set do
+    let!(:set) { described_class.new }
+
+    it 'concurrency' do
+      (1..THREADS).map do |i|
+        Thread.new do
+          1000.times do
+            v = i
+            set << v
+            expect(set).not_to be_empty
+            set.delete(v)
+          end
+        end
+      end.map(&:join)
+      expect(set).to be_empty
+    end
+  end
+end