Reliability is designed into an air turbine starter (ATS) by using a trade study model that accounts for time-dependent as well as random failure modes. An evaluation technique, based on the Weibull reliability distribution, that yields answers in terms of system mean time between failure, MTBF, is presented. A sample ATS design typical of those used for commercial jet aircraft main engine starting is analyzed. The resulting system MTBF agrees well with field data on similar units. The analysis also indicates that there is the potential for weight, envelope, and cost savings by redesigning the gears and bearings with respect to fatigue for a higher probability of failure with little effect on overall starter reliability.