This paper presents a means to determine in-service availability of systems and equipment in a rigorous and consistent manner so that the predicted reliabilities and/or levels of safety can be assured. In this way the reliability performance can be established as part of an overall system condition monitoring programme. In addition to validation of predicted system reliability performance, overall levels of safety can be improved by providing clear evidence to support reductions in the amount of intrusive maintenance activities. Ultimately it is possible to reduce ownership costs through optimising maintenance activities. (6 pages)