Manpower constitutes a vital element of the nation’s defense capability, one that must be sustained if tomorrow’s force is to be ready for the wide set of contingencies currently envisioned in the post-Cold War world. The authors focus on a key aspect of this concern: the effect of recent personnel tempo, or perstempo, on reenlistment. They develop several new measures of perstempo as well as construct a theoretical model of retention that encompasses the effects of perstempo. They report that limited episodes of long separation or hostile duty positively affect the decision to stay by first-term or early-career service members. However, more extensive duty, especially if it is hostile, can reduce this positive effect; in some cases, long or hazardous duty reduces reenlistment below what it would have been in the absence of such duty. The authors also recommend new methods of data collection and analysis, and propose future studies that would enhance retention.