In the history of construction materials used by humankind, Geosynthetic Clay Liners (GCLs) are relatively new, having been in use internationally since the late 1980s (Heerten 2002)^[1]. However in the author's (and many others') experience the way they work, and their advantages and limitations are not well understood by designers who have not studied them in depth, or who do not work with them often (Heerten & Koerner 2009)^[1]. Their international success over the last 30 years however, has recently led to a modern South African manufacturing plant, and this paper adopts a practical approach and examines the construction, applications and performance of these useful water and gas tight lining materials. The paper highlights the equivalence of GCLs to compacted clay linings (CCLs) and the substantial cost savings that their use can show over ‘conventional’ materials such as CCLs in structures including landfill liners and containment structures of many kinds. Their constraints are also noted. In addition, an independent generic specification (GIGSA 1200W)^[2] for GCLs is presented as a model non-commercial example of how to specify and construct linings incorporating these products.