Deterioration is a phenomenon which affects some systems by reducing its capability of fulfilling determined functions for which they were designed until finally, they fail. Even, some of them are affected by multiple degradation sources. Additionally, systems are not only subject to internal degradation, but also they can fail due to other causes. Shocks are also important failure mechanisms of some systems. Some of these shocks can be viewed as the accumulation of some random stress and other ones result in an immediate system failure.It defines maintenance strategy as the set of actions performed with the aim of preventing the system failures or, if it is not possible, reducing their consequences. Traditionally, the maintenance strategies are based on infinite life cycles. That means that, when a system fails, it can be replaced by a new and identical one an infinite number of times. However, this assumption seldom holds in practice since the technology does no remain constant and systems evolve.In this thesis, we propose, develop, analyse and compare new maintenance strategies based both in infinite life cycles and in finite life cycles, where the system only can be replaced a limited number of times. To this end, we consider systems subject to shocks and internal degradation, represented both for a unique and for multiple degradation processes, as well as the independence and dependence between both causes of failure.
