A repairable system is one which after failing to function properly may be restored to satisfactory working order by replacing or repairing certain components. The system will, throughout its lifetime, undergo alternating in-service times and repair/maintenance times. Repair/maintenance times may also be of interest, but it will be assumed initially that these are small relative to the in-service times. This chapter discusses the three functions of prime interest relating to repairable systems: the mean cumulative repair function (MCRF); the mean cumulative cost function (MCCF); and the rate of occurrence of failures (ROCOF). The ROCOF should not be confused with the hazard function. One way of describing the occurrence of failures is to state the mean time between successive failures, often known as the MTBF. The main difference between the MTBF and the mean time to failure, known as the MTTF, is in the usage.