This paper discusses the laboratory measurement of internal and interface shear strengths of geosynthetic clay liners (GCLs). All relevant issues are addressed, including test apparatus, gripping/clamping, hydration, consolidation, shear displacement rate, and post-test measurements. The standard 300 x 300 mm direct shear box is expected to remain the apparatus of choice for GCL strength testing. A poor gripping/clamping system may cause progressive failure of a GCL specimen, resulting in erroneous peak and large displacement shear strengths. GCL specimens should be hydrated and consolidated to match expected field hydration and loading conditions. Consolidation stresses should be applied in small increments to minimize bentonite extrusion. The appropriate displacement rate during shear is an issue of continuing debate. Available data indicate that dry encapsulated GCLs and hydrated geomembrane/needle-punched GCL interfaces can be sheared at 1 mm/min. Peak internal shear strengths of hydrated GCLs generally increase withincreasing displacement rate although some key studies have produced contradictory results. Residual internal shear strengths of hydrated GCLs may increase or remain constant with increasing displacement rate. A maximum displacement rate of 0.1 mm/min. is recommended for hydrated GCL internal shear tests until this issue is resolved. Once a test is completed, the mode of failure should be recorded and GCL water contents should be measured. Shear stress-displacement relationships should be included as part of all GCL testing reports.