The engineering of existing structures encompasses the examination of condition and load-carrying capacity, decision criteria, and methods for rehabilitation or reinforcement. This course presents the bases necessary for this approach at the level of materials, structural response, calculation, evaluation, and decision methods. It principally covers the case of reinforced and prestressed concrete structures. E. Denarié handles parts on deterioration mechanisms, condition evaluation, non-destructive testing, updating of resistances, reinforcement with new concretes, UHPFRC and external prestressing. A. Nussbaumer handles parts on updating of action effects, in depth examination: updating by semi-probabilistic analysis, required safety level, condition evaluation and reinforcement by means of glued CFRP lamellas.