The development of the use of geotextiles in earth structures implies the possibility of forecasting and calculating the behaviour of structures comprising geotextiles. This requires (1) a quantitative knowledge of the performance of the materials and (2) appropriate rules of use and calculation methods. For several years, the Comite Francais des Geotextiles (French Committee for Geotextiles) worked towards this aim. This paper presents the standardized methods adopted for measuring the mechanical and hydraulic characteristics of geotextiles (French standard AFNOR) and reviews the technical rules presently published and used for the selection of geotextiles in the construction of different types of earth structures. The durability of geotextiles was studied using samples from over 30 structures after years of service. The whole of the observations concerns the evolution in hydraulic and mechanical properties and shows the advantages of using geotextiles in long-term structures. For the covering abstract of the conference see IRRD 287689. (TRRL)