diff --git a/spec/fixtures/generated_proto2/TestOneofMessage.pb.cr b/spec/fixtures/generated_proto2/TestOneofMessage.pb.cr
new file mode 100644
index 0000000..e85e91f
--- /dev/null
+++ b/spec/fixtures/generated_proto2/TestOneofMessage.pb.cr
@@ -0,0 +1,25 @@
+#require "protobuf"
+
+module TestMessagesProto2
+
+  struct TestOneof1
+    include Protobuf::Message
+
+    contract do
+      optional :a, :int32, 1
+
+      optional :oo0_a, :int32, 2, oneof_index: 0
+      optional :oo0_b, :int32, 3, oneof_index: 0
+      optional :oo0_c, :int32, 4, oneof_index: 0
+
+      optional :b, :int32, 5
+
+      optional :oo1_a, :int32, 6, oneof_index: 1
+      optional :oo1_b, :int32, 7, oneof_index: 1
+      optional :oo1_c, :int32, 8, oneof_index: 1
+
+      oneof 0, "oo0"
+      oneof 1, "oo1"
+    end
+  end
+end
diff --git a/spec/fixtures/generated_proto3/TestOneofMessage.pb.cr b/spec/fixtures/generated_proto3/TestOneofMessage.pb.cr
new file mode 100644
index 0000000..2a2fd35
--- /dev/null
+++ b/spec/fixtures/generated_proto3/TestOneofMessage.pb.cr
@@ -0,0 +1,25 @@
+#require "protobuf"
+
+module TestMessagesProto3
+
+  struct TestOneof1
+    include Protobuf::Message
+
+    contract do
+      optional :a, :int32, 1
+
+      optional :oo0_a, :int32, 2, oneof_index: 0
+      optional :oo0_b, :int32, 3, oneof_index: 0
+      optional :oo0_c, :int32, 4, oneof_index: 0
+
+      optional :b, :int32, 5
+
+      optional :oo1_a, :int32, 6, oneof_index: 1
+      optional :oo1_b, :int32, 7, oneof_index: 1
+      optional :oo1_c, :int32, 8, oneof_index: 1
+
+      oneof 0, "oo0"
+      oneof 1, "oo1"
+    end
+  end
+end
diff --git a/spec/protobuf/message_spec.cr b/spec/protobuf/message_spec.cr
index 9ff0bc8..b2f7507 100644
--- a/spec/protobuf/message_spec.cr
+++ b/spec/protobuf/message_spec.cr
@@ -59,4 +59,96 @@ describe Protobuf::Message do
     test4 = TestMessagesProto2::Test4.new(d: [] of Int32)
     test4.to_protobuf.empty?.should be_true
   end
+
+  it "handles oneof in proto2" do
+    # When multiple oneof values are populated
+    # by the ctor only the first survives
+    msg = TestMessagesProto2::TestOneof1.new(
+      a: 1,
+      oo0_a: 2, oo0_b: 3, oo0_c: 4,
+      b: 5,
+      oo1_a: 6, oo1_b: 7, oo1_c: 8
+    )
+
+    payload = [ msg.a, msg.oo0_a, msg.oo0_b, msg.oo0_c, msg.b, msg.oo1_a, msg.oo1_b, msg.oo1_c ]
+    payload.should eq([1, 2, nil, nil, 5, 6, nil, nil])
+
+    # `msg.foo` tells us the string name
+    # of the member `foo` that is set
+    msg.oo0.should eq("oo0_a")
+    msg.oo1.should eq("oo1_a")
+
+    # Setting values outside of oneof's leaves the oneof's untouched
+    msg.a = 1
+    msg.b = 5
+    payload = [ msg.a, msg.oo0_a, msg.oo0_b, msg.oo0_c, msg.b, msg.oo1_a, msg.oo1_b, msg.oo1_c ]
+    payload.should eq([1, 2, nil, nil, 5, 6, nil, nil])
+
+    # Setting a oneof value resets the others in the same oneof
+    msg.oo0_b = 3
+    payload = [ msg.a, msg.oo0_a, msg.oo0_b, msg.oo0_c, msg.b, msg.oo1_a, msg.oo1_b, msg.oo1_c ]
+    payload.should eq([1, nil, 3, nil, 5, 6, nil, nil])
+    msg.oo0.should eq("oo0_b")
+
+    msg.oo1_b = 7
+    payload = [ msg.a, msg.oo0_a, msg.oo0_b, msg.oo0_c, msg.b, msg.oo1_a, msg.oo1_b, msg.oo1_c ]
+    payload.should eq([1, nil, 3, nil, 5, nil, 7, nil])
+    msg.oo0.should eq("oo0_b")
+    msg.oo1.should eq("oo1_b")
+  end
+
+  it "handles oneof in proto2 and proto3" do
+    [TestMessagesProto2::TestOneof1, TestMessagesProto3::TestOneof1].each do |message_class|
+      # When multiple oneof values are populated
+      # by the ctor only the first survives
+      msg = message_class.new(
+        a: 1,
+        oo0_a: 2, oo0_b: 3, oo0_c: 4,
+        b: 5,
+        oo1_a: 6, oo1_b: 7, oo1_c: 8
+      )
+
+      payload = [ msg.a, msg.oo0_a, msg.oo0_b, msg.oo0_c, msg.b, msg.oo1_a, msg.oo1_b, msg.oo1_c ]
+      payload.should eq([1, 2, nil, nil, 5, 6, nil, nil])
+
+      # `msg.foo` tells us the string name
+      # of the member `foo` that is set
+      msg.oo0.should eq("oo0_a")
+      msg.oo1.should eq("oo1_a")
+
+      # Setting values outside of oneof's leaves the oneof's untouched
+      msg.a = 1
+      msg.b = 5
+      payload = [ msg.a, msg.oo0_a, msg.oo0_b, msg.oo0_c, msg.b, msg.oo1_a, msg.oo1_b, msg.oo1_c ]
+      payload.should eq([1, 2, nil, nil, 5, 6, nil, nil])
+
+      # Setting a oneof value resets the others in the same oneof
+      msg.oo0_b = 3
+      payload = [ msg.a, msg.oo0_a, msg.oo0_b, msg.oo0_c, msg.b, msg.oo1_a, msg.oo1_b, msg.oo1_c ]
+      payload.should eq([1, nil, 3, nil, 5, 6, nil, nil])
+      msg.oo0.should eq("oo0_b")
+
+      msg.oo1_b = 7
+      payload = [ msg.a, msg.oo0_a, msg.oo0_b, msg.oo0_c, msg.b, msg.oo1_a, msg.oo1_b, msg.oo1_c ]
+      payload.should eq([1, nil, 3, nil, 5, nil, 7, nil])
+      msg.oo0.should eq("oo0_b")
+      msg.oo1.should eq("oo1_b")
+
+      # Serialization and deserialization through protobuf
+      deser_msg = msg.class.from_protobuf(msg.to_protobuf)
+      msg.should eq(deser_msg)
+
+      deser_msg.a.should eq(1)
+      deser_msg.oo0_a.should eq(nil)
+      deser_msg.oo0_b.should eq(3)
+      deser_msg.oo0_c.should eq(nil)
+
+      deser_msg.oo1_a.should eq(nil)
+      deser_msg.oo1_b.should eq(7)
+      deser_msg.oo1_c.should eq(nil)
+
+      deser_msg.oo0.should eq("oo0_b")
+      deser_msg.oo1.should eq("oo1_b")
+    end
+  end
 end
diff --git a/src/protobuf/generator.cr b/src/protobuf/generator.cr
index feb9636..2d54fd9 100644
--- a/src/protobuf/generator.cr
+++ b/src/protobuf/generator.cr
@@ -73,6 +73,33 @@ module Protobuf
         optional :default_value, :string, 7
 
         optional :options, FieldOptions, 8
+
+        ## If set, gives the index of a oneof in the containing type's oneof_decl
+        ## list.  This field is a member of that oneof.
+        optional :oneof_index, :int32, 9
+
+        ##  If true, this is a proto3 "optional". When a proto3 field is optional, it
+        ##  tracks presence regardless of field type.
+        ##
+        ##  When proto3_optional is true, this field must be belong to a oneof to
+        ##  signal to old proto3 clients that presence is tracked for this field. This
+        ##  oneof is known as a "synthetic" oneof, and this field must be its sole
+        ##  member (each proto3 optional field gets its own synthetic oneof). Synthetic
+        ##  oneofs exist in the descriptor only, and do not generate any API. Synthetic
+        ##  oneofs must be ordered after all "real" oneofs.
+        ##
+        ##  For message fields, proto3_optional doesn't create any semantic change,
+        ##  since non-repeated message fields always track presence. However it still
+        ##  indicates the semantic detail of whether the user wrote "optional" or not.
+        ##  This can be useful for round-tripping the .proto file. For consistency we
+        ##  give message fields a synthetic oneof also, even though it is not required
+        ##  to track presence. This is especially important because the parser can't
+        ##  tell if a field is a message or an enum, so it must always create a
+        ##  synthetic oneof.
+        ##
+        ##  Proto2 optional fields do not set this flag, because they already indicate
+        ##  optional with `LABEL_OPTIONAL`.
+        optional :proto3_optional, :bool, 17
       end
     end
 
@@ -84,6 +111,14 @@ module Protobuf
       end
     end
 
+    struct OneofDescriptorProto
+      include Protobuf::Message
+
+      contract do
+        optional :name, :string, 1
+      end
+    end
+
     struct EnumValueDescriptorProto
       include Protobuf::Message
 
@@ -184,6 +219,7 @@ module Protobuf
         repeated :extended,    CodeGeneratorRequest::FieldDescriptorProto, 6
         repeated :nested_type, CodeGeneratorRequest::DescriptorProto,      3
         repeated :enum_type,   CodeGeneratorRequest::EnumDescriptorProto,  4
+        repeated :oneof_decl,  CodeGeneratorRequest::OneofDescriptorProto, 8
       end
     end
 
@@ -236,6 +272,7 @@ module Protobuf
     end
 
     contract do
+      optional :supported_features, :uint64, 2
       repeated :file, CodeGeneratorResponse::File, 15
     end
   end
@@ -258,7 +295,9 @@ module Protobuf
           content: generator.compile
         )
       end
-      CodeGeneratorResponse.new(file: files)
+      # `supported_features` is explained here:
+      # https://github.com/protocolbuffers/protobuf/blob/master/src/google/protobuf/compiler/plugin.proto#L112
+      CodeGeneratorResponse.new(file: files, supported_features: 1)
     end
 
     @package_name : String?
@@ -329,10 +368,12 @@ module Protobuf
 
         syntax = @file.syntax.nil? ? "proto2" : @file.syntax
 
-        unless message_type.field.nil?
+        mt_field = message_type.field
+        unless mt_field.nil?
           puts "contract_of \"#{syntax}\" do"
           indent do
-            message_type.field.not_nil!.each { |f| field!(f, syntax) } unless message_type.field.nil?
+            mt_field.each { |f| field!(f, syntax) } # unless mt_field.nil?
+            message_type.oneof_decl.not_nil!.each_with_index { |oo, i| oneof!(oo, i) } unless message_type.oneof_decl.nil?
           end
           puts "end"
         end
@@ -340,6 +381,18 @@ module Protobuf
       puts "end"
     end
 
+    def oneof!(oneof_decl, index)
+      # This feels wrong. We rely on the the leading underscore
+      # to detect synthetic oneofs. This works but there must be
+      # a better way (yet to be discovered).
+      # https://github.com/protocolbuffers/protobuf/blob/master/docs/implementing_proto3_presence.md#updating-a-code-generator
+      if oneof_decl.name.not_nil![0] == '_'
+        puts "synthetic_oneof #{index}, \"#{oneof_decl.name}\""
+      else
+        puts "oneof #{index}, \"#{oneof_decl.name}\""
+      end
+    end
+
     def field!(field, syntax)
       met = case field.label
       when CodeGeneratorRequest::FieldDescriptorProto::Label::LABEL_OPTIONAL
@@ -396,6 +449,7 @@ module Protobuf
           field_desc += ", packed: true" if field.options.not_nil!.packed
         end
       end
+      field_desc += ", oneof_index: #{field.oneof_index}" if field.oneof_index && field.proto3_optional != true
       puts field_desc
     end
 
diff --git a/src/protobuf/message.cr b/src/protobuf/message.cr
index 6a0fb49..8972675 100644
--- a/src/protobuf/message.cr
+++ b/src/protobuf/message.cr
@@ -1,8 +1,8 @@
 module Protobuf
   module Message
-
     macro contract_of (syntax, &blk)
       FIELDS = {} of Int32 => HashLiteral(Symbol, ASTNode)
+      ONEOFS = {} of Int32 => String?
       {{yield}}
       _generate_decoder {{syntax}}
       _generate_encoder {{syntax}}
@@ -14,7 +14,7 @@ module Protobuf
       contract_of "proto2" {{blk}}
     end
 
-    macro _add_field(tag, name, pb_type, options = {} of Symbol => Bool)
+    macro _add_field(tag, name, pb_type, options = {} of Symbol => Bool, oneof_index = nil)
       {%
         t = ::Protobuf::PB_TYPE_MAP[pb_type] || pb_type
         FIELDS[tag] = {
@@ -27,22 +27,58 @@ module Protobuf
           repeated:     !!options[:repeated],
           default:      options[:default],
           packed:       !!options[:packed],
+          oneof_index:  oneof_index
         }
       %}
     end
 
-    macro optional(name, type, tag, default = nil, repeated = false, packed = false)
-      _add_field({{tag.id}}, {{name}}, {{type}}, {optional: true, default: {{default}}, repeated: {{repeated}}, packed: {{packed}}})
+    macro optional(name, type, tag, default = nil, repeated = false, packed = false, oneof_index = nil)
+      _add_field({{tag.id}}, {{name}}, {{type}},
+                 {optional: true, default: {{default}}, repeated: {{repeated}}, packed: {{packed}}}, oneof_index: {{oneof_index}})
     end
 
-    macro required(name, type, tag, default = nil)
-      _add_field({{tag.id}}, {{name}}, {{type}}, {default: {{default}}})
+    macro required(name, type, tag, default = nil, oneof_index = nil)
+      _add_field({{tag.id}}, {{name}}, {{type}}, {default: {{default}}}, oneof_index: {{oneof_index}})
     end
 
     macro repeated(name, type, tag, packed = false)
       optional({{name}}, {{type}}, {{tag}}, nil, true, {{packed}})
     end
 
+    macro synthetic_oneof(index, name)
+      # Synthetic oneofs don't get any special API,
+      # they are supposed to behave like proto2 optional fields.
+      #
+      # Leaving this macro in here as a placeholder regardless,
+      # in case special treatment will be needed later.
+    end
+
+    macro oneof(index, name)
+      {% ONEOFS[index] = name %}
+
+      @{{name.id}} : String?
+      def {{name.id}}
+        @{{name.id}}
+      end
+
+      def {{name.id}}!
+        @{{name.id}}.not_nil!
+      end
+
+      def {{name.id}}_value
+        {% for tag, field in FIELDS %}
+          {% if field[:oneof_index] == index %}
+            return @{{field[:name].id}} if @{{ONEOFS[index].id}} == {{ field[:name] }}.to_s
+          {% end %}
+        {% end %}
+        nil
+      end
+
+      def {{name.id}}_value!
+        {{name.id}}_value.not_nil!
+      end
+    end
+
     macro extensions(range)
       # puts "extensions: {{range.id}}"
     end
@@ -126,6 +162,10 @@ module Protobuf
             @{{field[:name].id}} = (%var{tag}).as({{field[:cast_type]}})
           {% end %}
         {% end %}
+
+        {% for tag, field in FIELDS %}
+          self.{{field[:name].id}} = @{{field[:name].id}}
+        {% end %}
       end
 
       def initialize(
@@ -140,6 +180,9 @@ module Protobuf
           {% end %}
         {% end %}
       )
+        {% for tag, field in FIELDS %}
+          self.{{field[:name].id}} = @{{field[:name].id}}
+        {% end %}
       end
     end
 
@@ -192,6 +235,27 @@ module Protobuf
     macro _generate_getters_setters
       {% for tag, field in FIELDS %}
         property {{field[:name].id}} : {{field[:cast_type]}}
+        def {{field[:name].id}}=(value : {{field[:cast_type]}})
+          @{{field[:name].id}} = value
+
+          # If this field is a member of a oneof..
+          {% unless field[:oneof_index] == nil %}
+            unless value.nil?
+              # ..we set the oneof to the name of the this field..
+              @{{ONEOFS[field[:oneof_index]].id}} = {{field[:name].id.stringify}}
+              # ..and clear the other members of the oneof
+              {% for neighbor_tag, neighbor_field in FIELDS %}
+                {% if neighbor_tag != tag && neighbor_field[:oneof_index] == field[:oneof_index] %}
+                  @{{neighbor_field[:name].id}} = {{neighbor_field[:default]}}
+                {% end %}
+              {% end %}
+            end
+          {% end %}
+        end
+
+        def {{field[:name].id}}!
+          @{{field[:name].id}}.not_nil!
+        end
       {% end %}
     end