This paper presents a unique comparison of single and multi-layer interface shear strength tests for a recently constructed landfill liner system. The comparison includes peak and large displacement combination strength envelopes from single- and multi-layer interface direct shear tests for the same geomembrane (GM)/ drainage geocomposite (GC), geosynthetic clay liner (GCL)/geomembrane, and soil/GCL interfaces. This comparison shows excellent agreement between strength envelopes and critical interfaces derived from single- and multi-layer interface tests for the materials tested. Based on this comparison, it is recommended that multi-layer interface tests be used for composite liner system design and if there is an anomaly with the multi-layer interface test results, single interface tests can be conducted to verify the weakest interface and to clarify the test results. Alternatively, the liner system can be designed using single interface tests and multi-layer interface tests can be used to verify the combination strength envelope derived from the single interface tests as was done in this case.