The chapter utilizes the continuous-time Markov chain for modeling the processes of the gradual aging with maintenance on a finite discrete set of an intermediate failure states. The transitions occur according to the birth-and-death process, and the unit fails completely after visiting the last available state. The unit of a multiple and single use is studied. The switching of the repair facility is performed by a hysteresis control policy with two threshold levels for switching on/off the repair server. We provide the expressions for the stationary and non-stationary performance and reliability characteristics, solution of optimization problems, and sensitivity analysis of the reliability function.