This paper provides guidelines for measuring the internal and interface shear strengths of geosynthetic clay liners (GCLs) in a direct shear test. Currently, there is significant variability between laboratories in shear testing procedures for GCLs; the major differences are related to specimen hydration and shear rate. The guidelines presented in this paper are intended to provide for consistent test results that can be interpreted without ambiguity. They are also intended to provide measured strengths that are representative of typical field conditions.