Fibre-reinforced polymer (FrP) composites are increasingly being used in the field of civil engineering, either for the rehabilitation/retrofitting of existing infrastructures or for the construction of new structural elements. However, such applications are still recent and there are still unresolved questions regarding the long-term durability of FrP reinforcements or structural elements under service conditions, and their behaviour under accidental fire events as well. In this chapter, it is proposed to highlight the basic mechanisms involved in the environmental degradation of FrP composites, with a large emphasis on ageing mechanisms of the polymer matrix and their consequences on the mechanical properties. the last section is specifically devoted to the fire behaviour of polymer composites and also recalls existing fire-proofing solutions.