The forces driving U.S. prison growth are poorly understood. This article examines one factor that has received insufficient attention: changes in time served. It demonstrates that time served has not risen dramatically in recent years, even declining in some jurisdictions. It also shows that time served is fairly short: median release times are approximately one to two years. Thus, admissions practices, not longer sentences, appear to drive prison growth. This article also examines whether time served varies across different types of inmates. Young, Hispanic, and violent offenders appear to serve longer sentences; race and sex appear to be of minor importance.