During the past decade, the use of geotextiles has increased significantly in a wide range of civil engineering applications: road construction, coastal engineering, stabilization of slopes and retaining structures, reinforcement of foundations, drainage etc. This work focuses on the factors which determine the level of long-term performance of these materials such as their mechanical and physical properties and physico-chemical ageing processes. (It includes contributions to an international seminar held at Saint-Remy-les-Chevreuse, France, in November 1986)