diff --git a/Gemfile b/Gemfile
index 90bd930..862f93f 100644
--- a/Gemfile
+++ b/Gemfile
@@ -39,8 +39,9 @@ group :test do
   gem "codeclimate-test-reporter",           require: false
 
   # test helpers
-  gem "fixture_builder",    "~> 0.4.1"
-  gem "factory_girl_rails", "~> 4.5.0"
+  gem "fixture_builder",           "~> 0.4.1"
+  gem "factory_girl_rails",        "~> 4.5.0"
+  gem "rspec-collection_matchers", "~> 1.1.2"
 end
 
 group :development, :test do
diff --git a/Gemfile.lock b/Gemfile.lock
index fc73be7..8edf793 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -167,6 +167,8 @@ GEM
       rspec-core (~> 3.3.0)
       rspec-expectations (~> 3.3.0)
       rspec-mocks (~> 3.3.0)
+    rspec-collection_matchers (1.1.2)
+      rspec-expectations (>= 2.99.0.beta1)
     rspec-core (3.3.1)
       rspec-support (~> 3.3.0)
     rspec-expectations (3.3.0)
@@ -238,6 +240,7 @@ DEPENDENCIES
   pry
   pry-byebug
   pry-debundle
+  rspec-collection_matchers (~> 1.1.2)
   rspec-rails (~> 3.3.2)
   rubocop (= 0.32.1)
   simplecov (= 0.10.0)
diff --git a/lib/diaspora_federation.rb b/lib/diaspora_federation.rb
index 3f1229b..1e4c546 100644
--- a/lib/diaspora_federation.rb
+++ b/lib/diaspora_federation.rb
@@ -1,6 +1,9 @@
 require "diaspora_federation/logging"
 
 require "diaspora_federation/callbacks"
+require "diaspora_federation/properties_dsl"
+require "diaspora_federation/entity"
+
 require "diaspora_federation/web_finger"
 
 # diaspora* federation library
diff --git a/lib/diaspora_federation/entity.rb b/lib/diaspora_federation/entity.rb
new file mode 100644
index 0000000..fa3793f
--- /dev/null
+++ b/lib/diaspora_federation/entity.rb
@@ -0,0 +1,146 @@
+module DiasporaFederation
+  # +Entity+ is the base class for all other objects used to encapsulate data
+  # for federation messages in the Diaspora* network.
+  # Entity fields are specified using a simple {PropertiesDSL DSL} as part of
+  # the class definition.
+  #
+  # Any entity also provides the means to serialize itself and all nested
+  # entities to XML (for deserialization from XML to +Entity+ instances, see
+  # {XmlPayload}).
+  #
+  # @abstract Subclass and specify properties to implement various entities.
+  #
+  # @example Entity subclass definition
+  #   class MyEntity < Entity
+  #     property :prop
+  #     property :optional, default: false
+  #     property :dynamic_default, default: -> { Time.now }
+  #     entity :nested, NestedEntity
+  #     entity :multiple, [OtherEntity]
+  #   end
+  #
+  # @example Entity instantiation
+  #   nentity = NestedEntity.new
+  #   oe1 = OtherEntity.new
+  #   oe2 = OtherEntity.new
+  #
+  #   entity = MyEntity.new(prop: 'some property',
+  #                         nested: nentity,
+  #                         multiple: [oe1, oe2])
+  #
+  # @note Entity properties can only be set during initialization, after that the
+  #   entity instance becomes frozen and must not be modified anymore. Instances
+  #   are intended to be immutable data containers, only.
+  class Entity
+    extend PropertiesDSL
+
+    # Initializes the Entity with the given attribute hash and freezes the created
+    # instance it returns.
+    #
+    # @note Attributes not defined as part of the class definition ({PropertiesDSL#property},
+    #       {PropertiesDSL#entity}) get discarded silently.
+    #
+    # @param [Hash] data
+    # @return [Entity] new instance
+    def initialize(data)
+      raise ArgumentError, "expected a Hash" unless data.is_a?(Hash)
+      missing_props = self.class.missing_props(data)
+      unless missing_props.empty?
+        raise ArgumentError, "missing required properties: #{missing_props.join(', ')}"
+      end
+
+      self.class.default_values.merge(data).each do |k, v|
+        instance_variable_set("@#{k}", v) if setable?(k, v)
+      end
+      freeze
+    end
+
+    # Returns a Hash representing this Entity (attributes => values)
+    # @return [Hash] entity data (mostly equal to the hash used for initialization).
+    def to_h
+      self.class.class_prop_names.each_with_object({}) do |prop, hash|
+        hash[prop] = send(prop)
+      end
+    end
+
+    # Returns the XML representation for this entity constructed out of
+    # {http://www.rubydoc.info/gems/nokogiri/Nokogiri/XML/Element Nokogiri::XML::Element}s
+    #
+    # @see Nokogiri::XML::Node.to_xml
+    # @see XmlPayload.pack
+    #
+    # @return [Nokogiri::XML::Element] root element containing properties as child elements
+    def to_xml
+      entity_xml
+    end
+
+    # some of this is from Rails "Inflector.demodulize" and "Inflector.undersore"
+    def self.entity_name
+      word = name.dup
+      i = word.rindex("::")
+      word = word[(i + 2)..-1] if i
+
+      word.gsub!("::", "/")
+      word.gsub!(/([A-Z\d]+)([A-Z][a-z])/, '\1_\2')
+      word.gsub!(/([a-z\d])([A-Z])/, '\1_\2')
+      word.tr!("-", "_")
+      word.downcase!
+      word
+    end
+
+    private
+
+    def setable?(name, val)
+      prop_def = self.class.class_props.find {|p| p[:name] == name }
+      return false if prop_def.nil? # property undefined
+
+      setable_string?(prop_def, val) || setable_nested?(prop_def, val) || setable_multi?(prop_def, val)
+    end
+
+    def setable_string?(definition, val)
+      (definition[:type] == String && val.respond_to?(:to_s))
+    end
+
+    def setable_nested?(definition, val)
+      t = definition[:type]
+      (t.is_a?(Class) && t.ancestors.include?(Entity) && val.is_a?(Entity))
+    end
+
+    def setable_multi?(definition, val)
+      t = definition[:type]
+      (t.instance_of?(Array) &&
+        val.instance_of?(Array) &&
+        val.all? {|v| v.instance_of?(t.first) })
+    end
+
+    # Serialize the Entity into XML elements
+    # @return [Nokogiri::XML::Element] root node
+    def entity_xml
+      doc = Nokogiri::XML::DocumentFragment.new(Nokogiri::XML::Document.new)
+      root_element = Nokogiri::XML::Element.new(self.class.entity_name, doc)
+
+      self.class.class_props.each do |prop_def|
+        name = prop_def[:name]
+        type = prop_def[:type]
+        if type == String
+          root_element << simple_node(doc, name)
+        else
+          # call #to_xml for each item and append to root
+          [*send(name)].compact.each do |item|
+            root_element << item.to_xml
+          end
+        end
+      end
+
+      root_element
+    end
+
+    # create simple node, fill it with text and append to root
+    def simple_node(doc, name)
+      node = Nokogiri::XML::Element.new(name.to_s, doc)
+      data = send(name).to_s
+      node.content = data unless data.empty?
+      node
+    end
+  end
+end
diff --git a/lib/diaspora_federation/properties_dsl.rb b/lib/diaspora_federation/properties_dsl.rb
new file mode 100644
index 0000000..16e84b7
--- /dev/null
+++ b/lib/diaspora_federation/properties_dsl.rb
@@ -0,0 +1,87 @@
+module DiasporaFederation
+  # Provides a simple DSL for specifying {Entity} properties during class
+  # definition.
+  module PropertiesDSL
+    # @return [Hash] hash of declared entity properties
+    def class_props
+      @class_props ||= []
+    end
+
+    # Define a generic (string-type) property
+    # @param [Symbol] name property name
+    # @param [Hash] opts further options
+    # @option opts [Object, #call] :default a default value, making the
+    #   property optional
+    def property(name, opts={})
+      define_property name, String, opts
+    end
+
+    # Define a property that should contain another Entity or an array of
+    # other Entities
+    # @param [Symbol] name property name
+    # @param [Entity, Array<Entity>] type Entity subclass or
+    #                Array with exactly one Entity subclass constant inside
+    # @param [Hash] opts further options
+    # @option opts [Object, #call] :default a default value, making the
+    #   property optional
+    def entity(name, type, opts={})
+      raise InvalidType unless type_valid?(type)
+
+      define_property name, type, opts
+    end
+
+    # Return array of missing required property names
+    def missing_props(args)
+      class_prop_names - default_props.keys - args.keys
+    end
+
+    # Return a new hash of default values, with dynamic values
+    # resolved on each call
+    def default_values
+      default_props.each_with_object({}) { |(name, prop), hash|
+        hash[name] = prop.respond_to?(:call) ? prop.call : prop
+      }
+    end
+
+    def nested_class_props
+      @nested_class_props ||= class_props.select {|p| p[:type] != String }
+    end
+
+    def class_prop_names
+      @class_prop_names ||= class_props.map {|p| p[:name] }
+    end
+
+    private
+
+    def define_property(name, type, opts={})
+      raise InvalidName unless name_valid?(name)
+
+      class_props << {name: name, type: type}
+      default_props[name] = opts[:default] if opts.has_key? :default
+
+      instance_eval { attr_reader name }
+    end
+
+    def name_valid?(name)
+      name.instance_of?(Symbol) || name.instance_of?(String)
+    end
+
+    def type_valid?(type)
+      [type].flatten.all? { |type|
+        type.respond_to?(:ancestors) && type.ancestors.include?(Entity)
+      }
+    end
+
+    def default_props
+      @default_props ||= {}
+    end
+
+    # Raised, if the name is of an unexpected type
+    class InvalidName < RuntimeError
+    end
+
+    # Raised, if the type is of an unexpected type
+    class InvalidType < RuntimeError
+    end
+  end
+end
diff --git a/spec/entities.rb b/spec/entities.rb
new file mode 100644
index 0000000..486a8a6
--- /dev/null
+++ b/spec/entities.rb
@@ -0,0 +1,22 @@
+module Entities
+  class TestEntity < DiasporaFederation::Entity
+    property :test
+  end
+
+  class TestDefaultEntity < DiasporaFederation::Entity
+    property :test1
+    property :test2
+    property :test3, default: true
+    property :test4, default: -> { true }
+  end
+
+  class OtherEntity < DiasporaFederation::Entity
+    property :asdf
+  end
+
+  class TestNestedEntity < DiasporaFederation::Entity
+    property :asdf
+    entity :test, TestEntity
+    entity :multi, [OtherEntity]
+  end
+end
diff --git a/spec/lib/diaspora_federation/entity_spec.rb b/spec/lib/diaspora_federation/entity_spec.rb
new file mode 100644
index 0000000..10192e5
--- /dev/null
+++ b/spec/lib/diaspora_federation/entity_spec.rb
@@ -0,0 +1,96 @@
+module DiasporaFederation
+  describe Entity do
+    let(:data) { {test1: "asdf", test2: 1234, test3: false, test4: false} }
+
+    it "should extend Entity" do
+      expect(Entities::TestDefaultEntity).to be < Entity
+    end
+
+    context "creation" do
+      it "freezes the instance after initialization" do
+        entity = Entities::TestDefaultEntity.new(data)
+        expect(entity).to be_frozen
+      end
+
+      it "checks for required properties" do
+        expect {
+          Entities::TestDefaultEntity.new({})
+        }.to raise_error ArgumentError, "missing required properties: test1, test2"
+      end
+
+      it "sets the defaults" do
+        entity = Entities::TestDefaultEntity.new(test1: 1, test2: 2)
+        expect(entity.to_h[:test3]).to be_truthy
+      end
+
+      it "handles callable defaults" do
+        entity = Entities::TestDefaultEntity.new(test1: 1, test2: 2)
+        expect(entity.to_h[:test4]).to be_truthy
+      end
+
+      it "uses provided values over defaults" do
+        entity = Entities::TestDefaultEntity.new(data)
+        expect(entity.to_h[:test3]).to be_falsey
+      end
+    end
+
+    describe "#to_h" do
+      it "returns a hash of the internal data" do
+        entity = Entities::TestDefaultEntity.new(data)
+        expect(entity.to_h).to eq(data)
+      end
+    end
+
+    describe "#to_xml" do
+      it "returns an Nokogiri::XML::Element" do
+        entity = Entities::TestDefaultEntity.new(data)
+        expect(entity.to_xml).to be_an_instance_of Nokogiri::XML::Element
+      end
+
+      it "has the root node named after the class (underscored)" do
+        entity = Entities::TestDefaultEntity.new(data)
+        expect(entity.to_xml.name).to eq("test_default_entity")
+      end
+
+      it "contains nodes for each of the properties" do
+        entity = Entities::TestDefaultEntity.new(data)
+        entity.to_xml.children.each do |node|
+          expect(%w(test1 test2 test3 test4)).to include(node.name)
+        end
+      end
+    end
+
+    describe ".entity_name" do
+      it "strips the module and returns the name underscored" do
+        expect(Entities::TestDefaultEntity.entity_name).to eq("test_default_entity")
+        expect(Entities::TestNestedEntity.entity_name).to eq("test_nested_entity")
+        expect(Entities::OtherEntity.entity_name).to eq("other_entity")
+      end
+    end
+
+    context "nested entities" do
+      let(:nested_data) {
+        {
+          asdf:  "FDSA",
+          test:  Entities::TestEntity.new(test: "test"),
+          multi: [Entities::OtherEntity.new(asdf: "asdf"), Entities::OtherEntity.new(asdf: "asdf")]
+        }
+      }
+
+      it "gets returned by #to_h" do
+        entity = Entities::TestNestedEntity.new(nested_data)
+        expect(entity.to_h).to eq(nested_data)
+      end
+
+      it "gets xml-ified by #to_xml" do
+        entity = Entities::TestNestedEntity.new(nested_data)
+        xml = entity.to_xml
+        xml.children.each do |node|
+          expect(%w(asdf test_entity other_entity)).to include(node.name)
+        end
+        expect(xml.xpath("test_entity")).to have_exactly(1).items
+        expect(xml.xpath("other_entity")).to have_exactly(2).items
+      end
+    end
+  end
+end
diff --git a/spec/lib/diaspora_federation/properties_dsl_spec.rb b/spec/lib/diaspora_federation/properties_dsl_spec.rb
new file mode 100644
index 0000000..8a225f2
--- /dev/null
+++ b/spec/lib/diaspora_federation/properties_dsl_spec.rb
@@ -0,0 +1,110 @@
+module DiasporaFederation
+  describe PropertiesDSL do
+    subject(:dsl) { Class.new.extend(PropertiesDSL) }
+
+    context "simple properties" do
+      it "can name simple properties by symbol" do
+        dsl.property :test
+        properties = dsl.class_props
+        expect(properties).to have(1).item
+        expect(properties.first[:name]).to eq(:test)
+        expect(properties.first[:type]).to eq(String)
+      end
+
+      it "can name simple properties by string" do
+        dsl.property "test"
+        properties = dsl.class_props
+        expect(properties).to have(1).item
+        expect(properties.first[:name]).to eq("test")
+        expect(properties.first[:type]).to eq(String)
+      end
+
+      it "will not accept other types for names" do
+        [1234, true, {}].each do |val|
+          expect {
+            dsl.property val
+          }.to raise_error PropertiesDSL::InvalidName
+        end
+      end
+
+      it "can define multiple properties" do
+        dsl.property :test
+        dsl.property :asdf
+        dsl.property :zzzz
+        properties = dsl.class_props
+        expect(properties).to have(3).items
+        expect(properties.map {|e| e[:name] }).to include(:test, :asdf, :zzzz)
+        properties.each {|e| expect(e[:type]).to eq(String) }
+      end
+    end
+
+    context "nested entities" do
+      it "gets included in the properties" do
+        expect(Entities::TestNestedEntity.class_prop_names).to include(:test, :multi)
+      end
+
+      it "can define nested entities" do
+        dsl.entity :other, Entities::TestEntity
+        properties = dsl.class_props
+        expect(properties).to have(1).item
+        expect(properties.first[:name]).to eq(:other)
+        expect(properties.first[:type]).to eq(Entities::TestEntity)
+      end
+
+      it "can define an array of a nested entity" do
+        dsl.entity :other, [Entities::TestEntity]
+        properties = dsl.class_props
+        expect(properties).to have(1).item
+        expect(properties.first[:name]).to eq(:other)
+        expect(properties.first[:type]).to be_an_instance_of(Array)
+        expect(properties.first[:type].first).to eq(Entities::TestEntity)
+      end
+
+      it "must be an entity subclass" do
+        [1234, true, {}].each do |val|
+          expect {
+            dsl.entity :fail, val
+          }.to raise_error PropertiesDSL::InvalidType
+        end
+      end
+
+      it "must be an entity subclass for array" do
+        [1234, true, {}].each do |val|
+          expect {
+            dsl.entity :fail, [val]
+          }.to raise_error PropertiesDSL::InvalidType
+        end
+      end
+    end
+
+    describe ".default_values" do
+      it "can accept default values" do
+        dsl.property :test, default: :foobar
+        defaults = dsl.default_values
+        expect(defaults[:test]).to eq(:foobar)
+      end
+
+      it "can accept default blocks" do
+        dsl.property :test, default: -> { "default" }
+        defaults = dsl.default_values
+        expect(defaults[:test]).to eq("default")
+      end
+    end
+
+    describe ".nested_class_props" do
+      it "returns the definition of nested class properties in an array" do
+        n_props = Entities::TestNestedEntity.nested_class_props
+        expect(n_props).to be_an_instance_of(Array)
+        expect(n_props.map {|p| p[:name] }).to include(:test, :multi)
+        expect(n_props.map {|p| p[:type] }).to include(Entities::TestEntity, [Entities::OtherEntity])
+      end
+    end
+
+    describe ".class_prop_names" do
+      it "returns the names of all class props in an array" do
+        expect(Entities::TestDefaultEntity.class_prop_names).to be_an_instance_of(Array)
+        expect(Entities::TestDefaultEntity.class_prop_names).to include(:test1, :test2, :test3, :test4)
+      end
+    end
+  end
+end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 2e97a0a..825cb21 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -22,6 +22,9 @@ require "rspec/rails"
 # load factory girl factories
 require "factories"
 
+# load test entities
+require "entities"
+
 # some helper methods
 
 def alice