Subtitle D landfills may experience elevated temperatures for a variety of reasons such as hydration of combustion ash, accelerated waste biodegradation, aluminum production waste, and hot wastes. Elevated temperatures can reduce service life and/or effectiveness of HDPE geomembranes by accelerating antioxidant depletion of geomembranes and polymer degradation. A case history is presented to illustrate the potential effect(s) of elevated temperatures and time-temperature history on a HDPE geomembrane and the associated reduction in service life and/or effectiveness. The geomembrane service life was influenced by the peak temperature, e.g., 60 to 80 °C, the duration of peak temperatures (time-temperature history), and the time to complete antioxidant depletion. This paper also discusses possible criteria for assessing the service life of geomembranes, such as, applicable engineering properties, locations for service life assessments, definitions for geomembrane service life, and measures that could be adopted if service life is reduced significantly. This paper highlights some of the questions that can arise when assessing the integrity of a composite liner system in the presence of sustained elevated temperatures. Based on experiences with landfills with elevated temperatures, the following conclusions and recommendations are presented based on data and the case history presented: (1) Heat generated in landfills can be produced from a variety of sources. The reaction of metallic aluminum in APW and leachate is one such source and data was presented to illustrate peak temperatures in an APW monofill range from 80 °C to 110 °C. In cases such as APW, the location of peak temperature in the landfill is dependent on availability of liquid/leachate and metallic aluminum and hence caution should be shown when landfilling reactive wastes in near proximity to the leachate collection system and composite liner system. (2) The presented case history shows temperatures at a MSW facility increased from normal operating conditions (35 °C to 45 °C) to elevated temperatures (70 °C to 85°C) due to APW reactions and smoldering combustion of MSW. Thermistors installed in leachate collection pipes were used to develop a time-temperature history plot to assess the service life of the geomembrane. For GM2, Cases 1 and 2 temperatures are in the range of normal MSW landfills; the geomembrane is expected to have a service life of several centuries. When peak temperatures reach 60 °C to 80 °C, the geomembrane service life can be reduced to decades for the conditions examined and thus raises concerns regarding the integrity of the geomembrane at high temperatures. (4) Although laboratory experiments evaluating the geomembrane activation energies used for Stages B and C have not been completed, the recommended criterion to estimate the service life of a geomembrane is the sum of time to complete antioxidant depletion (Stage A), Stage B, and time to stress cracking of HDPE geomembrane (Stage C). Using Arrhenius parameters for geomembranes determined via laboratory experiments and a time-temperature history plot, Stage A duration can be evaluated from Eq. (1) and (2). An approximation of Stage B and C can computed using Eq. (3) through (5). (5) Assuming proper construction and operations, the service life of a geomembrane liner used in a Subtitle D landfill will depend on temperature and time-temperature history, the chemical composition of the leachate, and the geomembrane properties. With respect to wide range of HDPE geomembranes, the resin and antioxidant package used may have a significant impact on the geomembrane's long-term performance in landfill applications. Therefore, the selection of an appropriate geomembrane is critical to the system's long term performance. The standard specification (GRI-GM13 1997) represents a basic starting point. While the minimum requirements may be sufficient for some applications, the specification GRI-GM13 may not be adequate for other applications such as elevated temperatures. Prior to geomembrane installation in landfills, laboratory experiments simulating higher temperatures and exposure conditions by the manufacturer or the designer are recommended to ensure adequate performance of the antioxidant package and resin. (6) Assessing the integrity of a composite liner and geomembrane is difficult and dependent on site geology and location of ground monitoring wells. For the design of new landfill cells, the barrier system should reflect the type of waste, e.g., MSW, APW, incinerator ash, etc., and mode of operation (dry cell, leachate recirculation, bioreactor). Installing a leak detection layer below the primary liner and thermistors to monitor the temperature provides direct monitoring of the liner performance. In addition, various strategies can be adopted to control the liner temperature (Hoor and Rowe 2012) and maintain an adequate geomembrane service life.