: The IP Multimedia Subsystem (IMS) is becoming the de facto overlay network for enabling the delivery of multimedia services in converged networks. Because it is envisioned that the IMS will experience a considerable growth in the years to come, it is important to verify that the involved resources conforming an IMS deployment deliver the desired performance when the system is operating under stress conditions. With the goal of gaining knowledge in the field of evaluation of such deployments, this paper introduces the IMS/NGN Performance Benchmark Specification developed by the ETSI, together with a detailed description of the results obtained when applied to an IMS testbed.