X‐band klystrons capable of 75 MW and utilizing either solenoidal or Periodic Permanent Magnet (PPM) focusing are undergoing design, fabrication and testing at the Stanford Linear Accelerator Center (SLAC). The klystron development is part of an effort to realize components necessary for the construction of the Next Linear Collider (NLC). SLAC has developed a solenoidal‐focused X‐band klystron which is currently the workhorse of high power component testing for the NLC. A state‐of‐the‐art modulator will drive eight of these tubes which, in turn, will power an rf distribution system referred to as the “8‐pack” in order to test these modulators and waveguide components. Eventually, in an interest to save millions of dollars per year in the operational cost of the NLC, these tubes will be replaced by PPM klystrons. The PPM devices built to date which fit this class of operation consist of a variety of 50 MW and 75 MW devices constructed by SLAC, KEK (Tsukuba, Japan ), and industry. These tubes follow from the successful 50 MW PPM design of 1996. Recent testing of this particular tube at wider pulsewidths has reached 50 MW at 55 &percent; efficiency, 2.4 &mgr;s and 60 Hz. Two 50 MW PPM klystrons produced by industry have been delivered to SLAC. One of these devices arrived with a vacuum suitable for test. Testing during 2001 revealed a serious, but curious, vacuum response which limited the operation to an rf output of ∼40 MW. A 75 MW PPM klystron prototype was first constructed in 1997 and later modified in 1999 to eliminate oscillations. This tube has reached the NLC design target of 75 MW at 1.5 &mgr;s though at a significantly reduced rep rate. Two new 75 MW PPM klystrons were constructed and tested in 2002 after a diode was successfully tested in 2001. The new design was aimed at reducing the cost and increasing the reliability of such high‐energy devices. The rf circuit and beam focusing for one of these devices was built by industry and incorporated into one of the tubes. Both of these latest devices suffered from a variety of issues concerning gun stability, beam confinement and rf stability. A rebuilt version of this latest design was constructed in early 2003 and completed testing in June. The performance of these various klystrons, particularly during 2002 and 2003, will be presented along with results of studies pertinent to their construction. Design and manufacturing issues of the various klystrons are discussed, along with plans for future modifications and areas of research. © 2003 American Institute of Physics