This chapter illustrates the use of geosynthetics, mainly geomembranes and geosynthetic clay liners, in hydraulic applications. After a brief history related to the use of geosynthetics in hydraulic applications, and the definition of the main terms related to geosynthetics used in this chapter, recommendations are given regarding the design of geosynthetic systems, taking into account the features of the underlying and overlying materials. Where necessary, a distinction is made between geomembranes and geocomposite clay liners, which have different features, even if both perform a lining function. A summary is then given of the durability of various types of geomembranes in hydraulic applications, based on the literature review. Finally, case studies of two ponds, three reservoirs and two canals are used to illustrate the recommendations.