A very frequently asked question regarding all types of geosynthetics is, ` How long will they last?' This paper answers the question for exposed geotextiles and geomembranes, assuming that they were properly designed and installed. Furthermore, it compares these new results to the earlier lifetime prediction results of a covered geomembrane. Nonexposed (or covered) lifetime conditions for a 1.5 mm thick high-density polyethylene (HDPE) geomembrane have previously been evaluated and published. Landfill incubation devices at four elevated temperatures of 85, 75, 65 and 55 degrees C were used in the prediction in order to reach 50% of retained strength and elongation. Considering the three stages of (i) depletion of antioxidants, (ii) induction time, and (iii) 50% reduction in mechanical properties, the lifetime extrapolation was made down to 20 degrees C. The 50% reduction value (called halflife throughout the paper) for this geomembrane under these conditions was approximately 450 years. Since the laboratory incubation times took 12 years, other nonexposed geosynthetics were not evaluated under the supposition that the covered situation is generally a moot point for most geosynthetics in their customary applications. For exposed (or uncovered) geosynthetics, however, the situation is quite different. Ultraviolet radiation, elevated temperature and full oxygen are available, which shortens the service lifetime, but how much? For evaluation of this situation, the authors utilized laboratory ultraviolet fluorescent tube weathering devices, as per ASTM D7238, for incubation purposes. Seven different geotextiles and five different geomembranes were evaluated. Each material was incubated at 80, 70 and 60 degrees C until a 50% reduction of strength and elongation occurred. The data was then extrapolated down to 20 degrees C for laboratory halflife values and for comparison with the nonexposed condition. The ratio of nonexposed to exposed lifetime for HDPE geomembranes is approximately seven. The calculations for the 12 exposed geosynthetics then progressed to using site-specific radiation to obtain an equivalent field halflife. Phoenix, Arizona, conditions are illustrated although the procedure is applicable worldwide. Halflife predictions for the geotextiles vary from a few months for the needle punched nonwovens to up to 10 years for monofilaments and high antioxidant formulated products. Results for geomembranes vary from 47 to 97 years, with HDPE being the highest. These exposed halflife results (which took 12 years of laboratory incubation to achieve) are felt to be most interesting and are presented for the first time to an international audience.