The concept and development of gyroklystron amplifiers with parameters relevant to providing rf driver tubes for TeV linear colliders is reviewed. Phase noise is shown to be acceptable for the collider application. Recent experimental results on a 3‐cavity, 9.87‐GHz gyroklystron (viz., 24 MW output power in a 1.7‐&mgr;s pulse with 36% efficiency and 33‐dB gain) are presented. These results establish the gyroklystron as a microwave amplifier with uniquely attractive capabilities. Prospects for scaling performance to still higher power, higher frequency and higher gain and efficiency are bright; advanced concepts for scaling are discussed.