The Spallation Neutron Source (SNS) is an accelerator‐based neutron source being built in Oak Ridge, Tennessee, by the U.S. Department of Energy. The SNS will provide the most intense pulsed neutron beams in the world for scientific research and industrial development. CPI is supporting the effort by providing 550 kW pulsed klystrons for the superconducting portion of the accelerator. A total of 73 units are on order. The primary output power requirements are 550 kW peak, 49.5 kW average at 805 MHz, with an electron beam‐to‐rf conversion efficiency of 65&percent; and an rf gain of 50 dB. To date, 25 units have been factory tested. Performance specifications, computer‐model predictions, and operating results are presented. © 2003 American Institute of Physics