diff --git a/README.md b/README.md
index 9e6dd7c..98bd22d 100644
--- a/README.md
+++ b/README.md
@@ -11,6 +11,11 @@ This module provides [Prometheus](https://prometheus.io/) metrics for
 it with your circuit breakers, just pass them in to the `PrometheusMetrics`
 constructor.
 
+For each circuit breaker, the metrics are:
+
+  * a `prometheus counter` for each event name
+  * a `prometheus summary` for the events `success`, `failed` execution time.
+
 Example:
 
 ```js
diff --git a/index.js b/index.js
index 8820282..02f5ede 100644
--- a/index.js
+++ b/index.js
@@ -23,6 +23,7 @@ class PrometheusMetrics {
     this._registry = registry || client.register;
     this._client = client;
     this.counters = [];
+    this.summaries = [];
 
     if (!registry) {
       this.interval = this._client
@@ -52,6 +53,20 @@ class PrometheusMetrics {
           counter.inc();
         });
         this.counters.push(counter);
+
+        if (eventName === 'success' || eventName === 'failure') {
+          // not the timeout event because runtime == timeout
+          const summary = new this._client.Summary({
+            name: `${prefix}${eventName}_perf`,
+            help:
+              `A summary of the ${circuit.name} circuit's ${eventName} event`,
+            registers: [this._registry]
+          });
+          circuit.on(eventName, (result, runTime) => {
+            summary.observe(runTime);
+          });
+          this.summaries.push(summary);
+        }
       }
     });
   }
diff --git a/test/prometheus-test.js b/test/prometheus-test.js
index 44d32b4..b664f98 100644
--- a/test/prometheus-test.js
+++ b/test/prometheus-test.js
@@ -31,30 +31,33 @@ test('The factory function accept no parameter', t => {
 });
 
 test('The factory function takes an object instead of just an Array', t => {
-  t.plan(2);
+  t.plan(3);
   const c1 = new CircuitBreaker(passFail, { name: 'fred' });
   const prometheus = new PrometheusMetrics(c1);
   t.equal(c1.name, 'fred');
   t.ok(/circuit_fred_/.test(prometheus.metrics));
+  t.ok(/circuit_fred_.*perf/.test(prometheus.metrics));
   prometheus.clear();
   t.end();
 });
 
 test('The factory function provides access to metrics for all circuits', t => {
-  t.plan(4);
+  t.plan(6);
   const c1 = new CircuitBreaker(passFail, { name: 'fred' });
   const c2 = new CircuitBreaker(passFail, { name: 'bob' });
   const prometheus = new PrometheusMetrics([c1, c2]);
   t.equal(c1.name, 'fred');
   t.equal(c2.name, 'bob');
   t.ok(/circuit_fred_/.test(prometheus.metrics));
+  t.ok(/circuit_fred_.*perf/.test(prometheus.metrics));
   t.ok(/circuit_bob_/.test(prometheus.metrics));
+  t.ok(/circuit_bob_.*perf/.test(prometheus.metrics));
   prometheus.clear();
   t.end();
 });
 
 test('The factory function uses a custom prom-client registry', t => {
-  t.plan(6);
+  t.plan(10);
   const registry = new Registry();
   const c1 = new CircuitBreaker(passFail, {
     name: 'fred'
@@ -66,26 +69,31 @@ test('The factory function uses a custom prom-client registry', t => {
   t.equal(c1.name, 'fred');
   t.equal(c2.name, 'bob');
   t.ok(/circuit_fred_/.test(registry.metrics()));
+  t.ok(/circuit_fred_.*perf/.test(registry.metrics()));
   t.ok(/circuit_bob_/.test(registry.metrics()));
+  t.ok(/circuit_bob_.*perf/.test(registry.metrics()));
   t.ok(/circuit_fred_/.test(prometheus.metrics));
+  t.ok(/circuit_fred_.*perf/.test(prometheus.metrics));
   t.ok(/circuit_bob_/.test(prometheus.metrics));
+  t.ok(/circuit_bob_.*perf/.test(prometheus.metrics));
   prometheus.clear();
   t.end();
 });
 
 test('The add function takes an object instead of just an Array', t => {
-  t.plan(2);
+  t.plan(3);
   const c1 = new CircuitBreaker(passFail, { name: 'fred' });
   const prometheus = new PrometheusMetrics();
   prometheus.add(c1);
   t.equal(c1.name, 'fred');
   t.ok(/circuit_fred_/.test(prometheus.metrics));
+  t.ok(/circuit_fred_.*perf/.test(prometheus.metrics));
   prometheus.clear();
   t.end();
 });
 
 test('The add function provides access to metrics for all circuits', t => {
-  t.plan(6);
+  t.plan(9);
   const c1 = new CircuitBreaker(passFail, { name: 'fred' });
   const c2 = new CircuitBreaker(passFail, { name: 'bob' });
   const c3 = new CircuitBreaker(passFail, { name: 'foo' });
@@ -95,13 +103,16 @@ test('The add function provides access to metrics for all circuits', t => {
   t.equal(c2.name, 'bob');
   t.equal(c3.name, 'foo');
   t.ok(/circuit_fred_/.test(prometheus.metrics));
+  t.ok(/circuit_fred_.*perf/.test(prometheus.metrics));
   t.ok(/circuit_bob_/.test(prometheus.metrics));
+  t.ok(/circuit_bob_.*perf/.test(prometheus.metrics));
   t.ok(/circuit_foo_/.test(prometheus.metrics));
+  t.ok(/circuit_foo_.*perf/.test(prometheus.metrics));
   prometheus.clear();
   t.end();
 });
 
-test('The add function accepts zero parameters', t => {
+test('The add function called without parameter do nothing', t => {
   t.plan(1);
   const prometheus = new PrometheusMetrics();
   prometheus.add();