diff --git a/lib/structural.rb b/lib/structural.rb
index 5929b88..68127ce 100644
--- a/lib/structural.rb
+++ b/lib/structural.rb
@@ -3,6 +3,7 @@
 
 require 'structural/version'
 require 'structural/missing_attribute_error'
+require 'structural/invalid_default_type_error'
 require 'structural/hashifier'
 require 'structural/model/definer'
 require 'structural/model/descriptor'
diff --git a/lib/structural/invalid_default_type_error.rb b/lib/structural/invalid_default_type_error.rb
new file mode 100644
index 0000000..760bff6
--- /dev/null
+++ b/lib/structural/invalid_default_type_error.rb
@@ -0,0 +1,4 @@
+module Structural
+  class InvalidDefaultTypeError < StandardError
+  end
+end
diff --git a/lib/structural/model/has_one.rb b/lib/structural/model/has_one.rb
index 015b9c3..a1300db 100644
--- a/lib/structural/model/has_one.rb
+++ b/lib/structural/model/has_one.rb
@@ -2,9 +2,22 @@ module Structural
   module Model
     class HasOne < Association
       def value_of(data)
-        child = data.fetch(key) { raise MissingAttributeError, key }
+        child = data.fetch(key, &default_value)
         type.new(child) unless child.nil?
       end
+
+      def default
+        valid_type_check(super)
+      end
+
+      private
+
+      def valid_type_check(v)
+        case v
+        when Hash then v
+        when Proc then valid_type_check(v.call)
+        else raise Structural::InvalidDefaultTypeError end
+      end
     end
   end
 end
diff --git a/lib/structural/version.rb b/lib/structural/version.rb
index 72e1437..b228c23 100644
--- a/lib/structural/version.rb
+++ b/lib/structural/version.rb
@@ -1,3 +1,3 @@
 module Structural
-  VERSION = '0.1.0'
+  VERSION = '0.2.0'
 end
diff --git a/spec/lib/structural/model_spec.rb b/spec/lib/structural/model_spec.rb
index 32476f9..32ba74b 100644
--- a/spec/lib/structural/model_spec.rb
+++ b/spec/lib/structural/model_spec.rb
@@ -3,6 +3,14 @@
 class NestedModel
   include Structural::Model
   field :yak
+  field :name, :default => nil
+  field :surname, :default => nil
+end
+
+class SeparateClass
+  def self.as_hash
+    { :name => 'Michael' }
+  end
 end
 
 class TestModel
@@ -20,6 +28,11 @@ class TestModel
   has_one :aliased_model, :type => NestedModel
   has_one :nested_model, :key => 'aliased_model'
   has_one :extra_nested_model
+  has_one :nested_with_invalid_default, :default => nil
+  has_one :nested_with_hash_default, :default => { name: 'Michael' }, :type => NestedModel
+  has_one :nested_with_hash_proc_default, :default => Proc.new { SeparateClass.as_hash }, :type => NestedModel
+  has_one :nested_with_invalid_proc_default, :default => Proc.new { SeparateClass.new }, :type => NestedModel
+  has_one :missing_nested_without_default
   has_one :test_model
   has_many :nested_models
 
@@ -87,20 +100,55 @@ class ExtraNestedModel
   end
 
   describe ".has_one" do
-    it "allows nested models" do
+    it 'allows nested models' do
       model.aliased_model.should be_a NestedModel
     end
-    it "allows nested models" do
+
+    it 'allows nested models' do
       model.nested_model.should be_a NestedModel
       model.nested_model.yak.should eq 11
     end
-    it "allows associations to be nested within the class" do
+
+    it 'allows associations to be nested within the class' do
       model.extra_nested_model.should be_a TestModel::ExtraNestedModel
       model.extra_nested_model.cats.should eq 'MIAOW'
     end
-    it "allows recursively defined models" do
+
+    it 'allows recursively defined models' do
       model.test_model.should be_a TestModel
     end
+
+    it 'allows default values as a hash' do
+      model.nested_with_hash_default.name.should eq 'Michael'
+      model.nested_with_hash_default.surname.should be_nil
+    end
+
+    context 'when passing a Proc as a default' do
+      it 'allows the default' do
+        model.nested_with_hash_proc_default.name.should eq 'Michael'
+        model.nested_with_hash_default.surname.should be_nil
+      end
+
+      context 'when the Proc does not return a Hash' do
+        it 'raises an error' do
+          expect {
+            model.nested_with_invalid_proc_default
+          }.to raise_error(Structural::InvalidDefaultTypeError)
+        end
+      end
+    end
+
+    it 'fails if passed a non-hash as default' do
+      expect {
+        model.nested_with_invalid_default
+      }.to raise_error(Structural::InvalidDefaultTypeError)
+    end
+
+    it 'fails for missing associations without defaults' do
+      expect {
+        model.missing_nested_without_default
+      }.to raise_error(Structural::MissingAttributeError)
+    end
   end
 
   describe ".has_many" do