An examination of some of the problems involved in measuring missile reliability during its prelaunch phase. Reliability is defined as the probability that an aggregate will successfully perform a specified task in a spedified environment over a specified period of time. Several of the most important prelaunch operating environments are: turn-on (-off), checkouts, countdowns, operating alert, standby alert, handling, transportation, periodic maintenance, and storage. The use of a missile status log is discussed in terms of measurement of missile-weapon-system reliability. A reliability measurement policy is suggested, and several difficulties of a reliability measurement system are noted.