The project summarized here focused on the stress cracking behavior and testing of geomembranes made from high-density polyethylene (HDPE). The project included a literature study of stress cracking failures in the field and exhumation of gemembranes at various sites of opportunity. In total, twenty-five cases of stress cracking have been documented since the introduction of this particular material in 1980. Because of the limitations of the currently used ASTM D 1693 "bent strip" test method for evaluating stress cracking behavior of HDPE geomembranes, a new test was developed-the notched, constant-tensile-load (NCTL) test. Eighteen commericially available sheet materials and seven field-exhumed geomembranes were evaluated using the test. The results led to the recommendation that for an HDPE geomembrane to be acceptably crack resistant, the transition time to the onset of brittle behavior must be greater than 100 hr. The results also indicated that the NCTL test is much more challenging than is the bent strip test. In addition, two related tests, a single-point, notched, constant-tensile-load (SP-NCTL) test and a seam, constant-tensile-load (SCTL) test was establised. The recommendations for both of these tests were that no failure should occur within 200 hr at an applied stress of 30% of yield stress of the sheet for five replicate tests. The implementation of this set of three tests is oriented toward resin suppliers, manufacturers, designers, and owners involved with HDPE geomembranes. By employing the recommendations made in the full report, the incidence of stress cracking of HDPE geomembranes in the field should be significantly reduced.