The objective of this program is to develop a gyrotron oscillator capable of producing 100 kW CW at 140 GHz. Several sets of calculations are described characterizing the TE/sub 021//TE/sub 031/ complex cavity used on Preprototype Tube 1. Initial design calculations for a new complex cavity design for Preprototype Tube 2 are summarized. Several analyses of the output window design were performed to improve agreement between calculated and meaured temperature data and to investigate the behavior of the window design at higher power levels. The results of these analyses are reported. Long-pulse and CW tests on Preprototype Tube 1 were carried out. During these tests, a CW power level of 100 kW was obtained, thus meeting the major technical goal of the 140 GHz Gyrotron Development Program. Long-pulse tests resulted in the achievement of a power level of 150 kW during 100 ms pulse operation. Additional short-pulse tests yielded power levels in excess of 200 kW. The results of the various tube tests on Preprototype Tube 1 are described and compared to theoretical predictions where possible. Initial tube tests on the rebuild of Experimental Tube 1 are summarized.