The primary goal of the study was to establish criteria for designing high power microwave amplifiers capable of adequately functioning in environments associated with space missions. The expected space environment, both natural and artificial, was carefully catalogued and a special treatment for calculating the ionizing radiation of the Van Allen Belts developed, including assumptions and their degree of uncertainty. The effects of the over- all environment on the materials of a space tube package were evaluated and the materials given a preference ranking. The essential properties required of a space tube are also enumerated and means of achieving them discussed, in detail, for linear-beam tubes, especially for traveling-wave amplifiers required to operate in the range between 1 Gc at 10,000 watts CW and 10 Gc at 100 watts CW. Limitations and problem areas are pointed out and some new concepts are advanced. The importance of pre-flight testing is emphasized, and a philosophy of combined-environment testing advanced sequential testing is also discussed.