diff --git a/lib/vanity.rb b/lib/vanity.rb
index 3e097562..31cf62ed 100644
--- a/lib/vanity.rb
+++ b/lib/vanity.rb
@@ -1,6 +1,7 @@
 $LOAD_PATH.unshift File.join(File.dirname(__FILE__), "../vendor/redis-0.1/lib")
 require "redis"
 require "openssl"
+require "date"
 
 # All the cool stuff happens in other places:
 # - Vanity::Helpers
@@ -18,6 +19,7 @@ module Version
 end
 
 require "vanity/playground"
+require "vanity/metric"
 require "vanity/experiment/base"
 require "vanity/experiment/ab_test"
 require "vanity/rails" if defined?(Rails)
diff --git a/lib/vanity/metric.rb b/lib/vanity/metric.rb
new file mode 100644
index 00000000..1e9ada01
--- /dev/null
+++ b/lib/vanity/metric.rb
@@ -0,0 +1,113 @@
+module Vanity
+
+  # A metric is an object with a method called values, which accepts two
+  # arguments, start data and end date, and returns an array of measurements.
+  #
+  # A metric can also respons to additional methods (track!, bounds, etc).
+  # This class implements a metric, use this as reference to the methods you
+  # can implement in your own metric.
+  #
+  # Or just use this metric implementation.  It's fast and fully functional.
+  # 
+  # Startup metrics for pirates: AARRR stands for Acquisition, Activation,
+  # Retention, Referral and Revenue.
+  # http://500hats.typepad.com/500blogs/2007/09/startup-metrics.html
+  class Metric
+
+    class << self
+
+      # Helper method to return title for a metric.
+      #
+      # A metric object may have a +title+ method that returns a short
+      # descriptive title.  It may also have no title, or no +title+ method, in
+      # which case the metric identifier will do.
+      # 
+      # Example:
+      #   Vanity.playground.metrics.map { |id, metric| Vanity::Metric.title(id, metric) }
+      def title(id, metric)
+        metric.respond_to?(:title) && metric.title || id.to_s.capitalize.gsub(/_+/, " ")
+      end
+
+      # Helper method to return description for a metric.
+      #
+      # A metric object may have a +description+ method that returns a detailed
+      # description.  It may also have no description, or no +description+
+      # method, in which case return +nil+.
+      # 
+      # Example:
+      #   puts Vanity::Metric.description(metric)
+      def description(metric)
+        metric.description if metric.respond_to?(:description)
+      end
+
+      # Helper method to return bounds for a metric.
+      #
+      # A metric object may have a +bounds+ method that returns lower and upper
+      # bounds.  It may also have no bounds, or no +bounds+ # method, in which
+      # case we return +[nil, nil]+.
+      # 
+      # Example:
+      #   upper = Vanity::Metric.bounds(metric).last
+      def bounds(metric)
+        metric.respond_to?(:bounds) && metric.bounds || [nil, nil]
+      end
+
+    end
+
+    def initialize(playground, id)
+      @playground = playground
+      @id = id
+      @hooks = []
+    end
+
+    # All metrics implement this value.  Given two arguments, a start date and
+    # an end date, it returns an array of measurements.
+    def values(to, from)
+      @playground.redis.mget((to.to_date..from.to_date).map { |date| "metrics:#{id}:#{date}:count" }).map(&:to_i)
+    end
+
+    # This method returns the acceptable bounds of a metric as an array with
+    # two values: low and high.  Use nil for unbounded.
+    #
+    # Alerts are created when metric values exceed their bounds.  For example,
+    # a metric of user registration can use historical data to calculate
+    # expected range of new registration for the next day.  If actual metric
+    # falls below the expected range, it could indicate registration process is
+    # broken.  Going above higher bound could trigger opening a Champagne
+    # bottle.
+    #
+    # The default implementation returns +nil+.
+    def bounds
+    end
+
+    # Metric identifier.
+    attr_accessor :id
+
+    # Metric title.
+    attr_accessor :title
+
+    # Metric description.
+    attr_accessor :description
+
+    # Called to track an action associated with this metric.
+    def track!(vanity_id)
+      timestamp = Time.now
+      @playground.redis.incr "metrics:#{id}:#{timestamp.to_date}:count"
+      @playground.logger.info "vanity tracked #{title || id}"
+      @hooks.each do |hook|
+        hook.call id, timestamp, vanity_id
+      end
+    end
+
+    # Metric definitions use this to introduce tracking hook.  The hook is
+    # called with three arguments: metric id, timestamp and vanity identity.
+    #
+    # For example:
+    #   hook do |metric_id, timestamp, vanity_id|
+    #     syslog.info metric_id
+    #   end
+    def hook(&block)
+      @hooks << block
+    end
+  end
+end
diff --git a/lib/vanity/playground.rb b/lib/vanity/playground.rb
index 5562b4b4..664d63b9 100644
--- a/lib/vanity/playground.rb
+++ b/lib/vanity/playground.rb
@@ -27,9 +27,12 @@ class Playground
     # Created new Playground. Unless you need to, use the global Vanity.playground.
     def initialize
       @experiments = {}
+      @metrics = {}
       @host, @port, @db = "127.0.0.1", 6379, 0
       @namespace = "vanity:#{Vanity::Version::MAJOR}"
       @load_path = "experiments"
+      @logger = Logger.new(STDOUT)
+      @logger.level = Logger::ERROR
     end
     
     # Redis host name.  Default is 127.0.0.1
@@ -117,6 +120,22 @@ class << self ; self ; end.send(:define_method, :redis) { redis }
       redis
     end
 
+    # Returns a metric (creating one if doesn't already exist).
+    def metric(id)
+      id = id.to_sym
+      @metrics[id] ||= Metric.new(self, id)
+    end
+
+    # Returns hash of metrics (key is metric id).
+    def metrics
+      @metrics
+    end
+
+    # Tracks an action associated with a metric.  For example:
+    #   Vanity.playground.track! :uploaded_video
+    def track!(id)
+      metric(id).track! Vanity.context.vanity_identity
+    end
   end
 
   @playground = Playground.new
diff --git a/lib/vanity/rails/helpers.rb b/lib/vanity/rails/helpers.rb
index 84bd573e..2cc04310 100644
--- a/lib/vanity/rails/helpers.rb
+++ b/lib/vanity/rails/helpers.rb
@@ -103,8 +103,8 @@ def ab_test(name, &block)
     #     track! :call_to_action
     #     Acccount.create! params[:account]
     #   end
-    def track!(name, *args)
-      Vanity.playground.experiment(name).track! *args
+    def track!(name)
+      Vanity.playground.track! name
     end
   end
 end
diff --git a/test/metric_test.rb b/test/metric_test.rb
new file mode 100644
index 00000000..6ef704b9
--- /dev/null
+++ b/test/metric_test.rb
@@ -0,0 +1,141 @@
+require "test/test_helper"
+
+class MetricTest < MiniTest::Unit::TestCase
+  def setup
+    super
+    Vanity.context = mock("Context")
+    Vanity.context.stubs(:vanity_identity).returns(rand)
+  end
+  
+  # -- Via the playground --
+
+  def test_playground_creates_metric_on_demand
+    assert metric = Vanity.playground.metric(:on_demand)
+    assert_equal :on_demand, metric.id
+  end
+
+  def test_playground_tracks_all_loaded_metrics
+    Vanity.playground.metric(:work)
+    Vanity.playground.metric(:play)
+    assert_includes Vanity.playground.metrics.keys, :play
+    assert_includes Vanity.playground.metrics.keys, :work
+  end
+
+  def test_playground_tracking_creates_metric_on_demand
+    Vanity.playground.track! :on_demand
+    assert_includes Vanity.playground.metrics.keys, :on_demand
+    assert_respond_to Vanity.playground.metrics[:on_demand], :values
+  end
+
+
+  # -- Tracking --
+
+  def test_tracking_can_count
+    4.times { Vanity.playground.track! :play }
+    2.times { Vanity.playground.track! :work }
+    play = Vanity.playground.metric(:play).values(Date.today, Date.today).first
+    work = Vanity.playground.metric(:work).values(Date.today, Date.today).first
+    assert play = 2 * work
+  end
+
+  def test_tracking_can_tell_the_time
+    Time.is (Date.today - 4).to_time do
+      4.times { Vanity.playground.track! :play }
+    end
+    Time.is (Date.today - 2).to_time do
+      2.times { Vanity.playground.track! :play }
+    end
+    1.times { Vanity.playground.track! :play }
+    values = Vanity.playground.metric(:play).values(Date.today - 5, Date.today)
+    assert_equal [0,4,0,2,0,1], values
+  end
+
+
+  # -- Tracking and hooks --
+
+  def test_tracking_runs_hook
+    returns = 0
+    Vanity.playground.metric(:many_happy_returns).hook do |metric_id, timestamp, vanity_id|
+      assert_equal :many_happy_returns, metric_id
+      assert_in_delta Time.now.to_i, timestamp.to_i, 1
+      assert_equal Vanity.context.vanity_identity, vanity_id
+      returns += 1
+    end
+    Vanity.playground.track! :many_happy_returns
+    assert_equal 1, returns
+  end
+
+  def test_tracking_runs_multiple_hooks
+    returns = 0
+    Vanity.playground.metric(:many_happy_returns).hook { returns += 1 }
+    Vanity.playground.metric(:many_happy_returns).hook { returns += 1 }
+    Vanity.playground.metric(:many_happy_returns).hook { returns += 1 }
+    Vanity.playground.track! :many_happy_returns
+    assert_equal 3, returns
+  end
+
+
+  # -- Title helper --
+  
+  def test_title_for_metric_with_title
+    metric = Vanity.playground.metric(:bst)
+    metric.title = "Blood, sweat, tears"
+    assert_equal "Blood, sweat, tears", Vanity::Metric.title(:bst, metric)
+  end
+
+  def test_title_for_metric_with_no_title
+    metric = Vanity.playground.metric(:bst)
+    assert_equal "Bst", Vanity::Metric.title(:bst, metric)
+  end
+
+  def test_title_for_metric_with_no_title_attributes
+    metric = Object.new
+    assert_equal "Bst", Vanity::Metric.title(:bst, metric)
+  end
+
+  def test_title_for_metric_with_compound_id
+    metric = Vanity.playground.metric(:blood_sweat_tears)
+    assert_equal "Blood sweat tears", Vanity::Metric.title(:blood_sweat_tears, metric)
+  end
+
+
+  # -- Description helper --
+
+  def test_description_for_metric_with_description
+    metric = Vanity.playground.metric(:bst)
+    metric.description = "I didn't say it will be easy"
+    assert_equal "I didn't say it will be easy", Vanity::Metric.description(metric)
+  end
+
+  def test_description_for_metric_with_no_description
+    metric = Vanity.playground.metric(:bst)
+    assert_nil Vanity::Metric.description(metric)
+  end
+
+  def test_description_for_metric_with_no_description_method
+    metric = Object.new
+    assert_nil Vanity::Metric.description(metric)
+  end
+
+
+  # -- Metric bounds --
+
+  def test_bounds_helper_for_metric_with_bounds
+    metric = Vanity.playground.metric(:eggs)
+    metric.instance_eval do
+      def bounds ; [6,12] ; end
+    end
+    assert_equal [6,12], Vanity::Metric.bounds(metric)
+  end
+
+  def test_bounds_helper_for_metric_with_no_bounds
+    metric = Vanity.playground.metric(:sky_is_limit)
+    assert_equal [nil, nil], Vanity::Metric.bounds(metric)
+  end
+
+  def test_bounds_helper_for_metric_with_no_bounds_method
+    metric = Object.new
+    assert_equal [nil, nil], Vanity::Metric.bounds(metric)
+  end
+
+end
diff --git a/test/mock_redis.rb b/test/mock_redis.rb
new file mode 100644
index 00000000..4f77b17c
--- /dev/null
+++ b/test/mock_redis.rb
@@ -0,0 +1,62 @@
+# The Redis you should never use in production.
+class MockRedis
+  @@hash = {}
+
+  def initialize(options)
+  end
+
+  def [](key)
+    @@hash[key]
+  end
+
+  def []=(key, value)
+    @@hash[key] = value.to_s
+  end
+
+  def del(key)
+    @@hash.delete key
+  end
+
+  def setnx(key, value)
+    @@hash[key] = value.to_s unless @@hash.has_key?(key)
+  end
+
+  def incr(key)
+    @@hash[key] = (@@hash[key].to_i + 1).to_s
+  end
+
+  def mget(keys)
+    @@hash.values_at(*keys)
+  end
+
+  def flushdb
+    @@hash.clear
+  end
+
+  def sismember(key, value)
+    case set = @@hash[key]
+    when nil ; false
+    when Set ; set.member?(value)
+    else fail "Not a set"
+    end
+  end
+
+  def sadd(key, value)
+    case set = @@hash[key]
+    when nil ; @@hash[key] = Set.new([value])
+    when Set ; set.add value
+    else fail "Not a set"
+    end
+  end
+
+  def scard(key)
+    case set = @@hash[key]
+    when nil ; 0
+    when Set ; set.size
+    else fail "Not a set"
+    end
+  end
+end
+
+# Use mock redis.
+Object.const_set :Redis, MockRedis
diff --git a/test/test_helper.rb b/test/test_helper.rb
index 3e86043c..1f5de095 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -7,9 +7,11 @@
 require "action_controller/test_case"
 require "initializer"
 require "lib/vanity/rails"
+require "test/mock_redis" # <-- load this when you don't want to use Redis
 MiniTest::Unit.autorun
 
 class MiniTest::Unit::TestCase
+
   # Call this on teardown. It wipes put the playground and any state held in it
   # (mostly experiments), resets vanity ID, and clears Redis of all experiments.
   def nuke_playground
@@ -33,3 +35,24 @@ def teardown
   map.connect ':controller/:action/:id'
 end
 Rails.configuration = Rails::Configuration.new
+
+
+# Time.now adapted from Jason Earl:
+# http://jasonearl.com/blog/testing_time_dependent_code/index.html
+def Time.now
+  @active || new
+end
+    
+# Set the time to be fake for a given block of code
+def Time.is(new_time, &block)
+  if block_given?
+    begin
+      old_time, @active = @active, new_time
+      yield
+    ensure
+      @active = old_time
+    end
+  else
+    @active = new_time || Time.new
+  end
+end