This paper presents on-orbit performance of the CloudSat extended interaction klystron (EIK) and reviews requirements, design and performance of EIK's for future space missions. The CloudSat mission provides cross-sectional view of clouds with information on their thickness, altitude, optical properties, water and ice content. It is concluded that following successful development and on-orbit operation of the CloudSat EIK, more flight models of W-band and Ka-band EIK's are currently under development.