This book deals to stochastic modelling and the parametric estimation of the maintenance efficiency for industrial systems in degradation. The assessing criterion of the maintenance efficiency is supported with the failure intensity in the bath-tub form. The study is devoted to the reformulation of the Brown-Proschan model, and that of models called arithmetic reduction of intensity and age. Relevant results relating to the theoretical properties of the maintenance efficiency estimators, based on the simulation method, allow to better understand the system behaviour. Ultimately, a method assessing of the efficiency relating to the total process of corrective and preventive maintenances is developed and illustrated on a basis of real data issued from industry