diff --git a/lib/rubella/storage.rb b/lib/rubella/storage.rb
index 989af2b..7a736b2 100644
--- a/lib/rubella/storage.rb
+++ b/lib/rubella/storage.rb
@@ -49,6 +49,8 @@ module Rubella
     # Returns the current length of the storage.
     # (How many datasets it holds.)
     #
+    # TODO Throw an error, if data has different lengths
+    #
     # @return Integer length
     #
     def length
diff --git a/spec/unit/rubella/storage/dataset_length_spec.rb b/spec/unit/rubella/storage/dataset_length_spec.rb
new file mode 100644
index 0000000..624dbb4
--- /dev/null
+++ b/spec/unit/rubella/storage/dataset_length_spec.rb
@@ -0,0 +1,29 @@
+require 'spec_helper'
+
+describe Rubella::Storage, '.dataset_length' do
+
+  it "returns 0 if Storage is empty" do
+    storage = Rubella::Storage.new []
+
+    expect(storage.dataset_length).to eq(0)
+  end
+
+  it "returns 1 if Storage holds one dimensional array" do
+    storage = Rubella::Storage.new [1, 2, 3]
+
+    expect(storage.dataset_length).to eq(1)
+  end
+
+  it "returns 1 if Storage datasets have a length of 1" do
+    storage = Rubella::Storage.new [[1], [2], [3]]
+
+    expect(storage.dataset_length).to eq(1)
+  end
+
+  it "returns the length of the datasets" do
+    storage = Rubella::Storage.new [[1, 2], [3, 4], [5, 6]]
+
+    expect(storage.dataset_length).to eq(2)
+  end
+
+end