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The question of Traveling Wave Tube (TWT) reliability in space poses some unique problems. First, since tube reliability has a tremendous impact on system design and overall cost, if problems do occur, they are highly visible. Second, attaining high reliability is made difficult by small production runs and short delivery schedules. Finally, the now-common 10 year life specification is combined with state-of-the-art performance requirements, forcing design changes and adding risk. To meet these requirements, we emphasize certain design and manufacturing ground rules. When orbital TWT problems do occur, our experience is that they are usually caused by infant mortality, not wearout. Data based on operation in space show that with close attention to the details of design and manufacturing, reliability exceeding the 500,000 hours MTBF normally specified is achieved. Traveling Wave Tube reliability and overall performance have a tremendous impact on system design and the overall cost of a satellite. TWT reliability determines the amount of redundancy needed to meet a given satellite mission objective. Increased redundancy means increased complexity and weight of the spacecraft. The TWTs, with their Electronic Power Conditioners, also dissipate over 80% of all spacecraft power. Increased tube efficiency will therefore simplify matters all around. Because of the critical impact of tubes, if technical problems do occur, they are highly visible at the system level, where rumors of failure spread like wildfire in the fairly small space community. Attaining high reliability is difficult because of the many conflicting requirements.