Abstract : Today, maintenance strategies and their analyses remain a worrying problem forcompanies. Socio economic stakes depending on competitiveness of each strategy are even morelinked to activity and quality of maintenance interventions. A series of specific events caneventually warn the expert of an imminent breakdown. Our study aims at understanding this"signature" thanks to an hidden Markov model. On that purpose, two strategies on damage levelestimation of maintained system are proposed to expert. The first one consists in using of nonparametric and semi parametric degradation laws. The second one consists in using of anmarkovian approach. All proposals are illustrated on two case studies corresponding to two realindustrial situations (continuous system for food processing and moulded products in aluminumalloys in an discontinuous process).