Increased environmental awareness and recent changes in US environmental regulations have forced many existing landfill sites to close. The owners of these sites want a simple and affordable capping system that will also allow them to meet the new stricter regulations. Geosynthetic clay liners (GCLs) provide an attractive alternative to traditional compacted clay liners (CCLs) as the barrier layer in a final cover system. GCLs provide many performance advantages including better freeze-thaw and wet-dry behavior and greater tolerance of differential settlement. GCLs are also much easier to install than traditional CCLs. This paper focuses on GCL design considerations of particular importance to caps and highlights two projects in the United States where GCLs were used as the sole barrier layer in the final cover system.