Movement of fluids in the unsaturated zone plays an important role in many geoenvironrnental engineering problems. Examples include cover and basal liner systems for waste containment facilities where geosynthetics are widely used, amongst many other examples. This paper highlights the importance of assessing the unsaturated characteristics of geosynthetics and their influence on the behaviour of engineered systems where soils and geosynthetics interact under unsaturated conditions. It includes information on the water retention curve and hydraulic conductivity function of geosynthetics such as geotextiles and geosynthetic clay liners (GCLs) with particular focus on capillary barriers, liner performance under elevated temperatures, and interface friction respectively. Mechanisms involved in the development of capillary barriers are evaluated to explain the storage of water at the interface between materials with contrasting hydraulic conductivity (e.g. a fine-grained soil and a nonwoven geotextile). Potential desiccation of GCLs is explained in the light of an application in a liquid waste impoundment. (C) 2013 Elsevier B.V. All rights reserved.