Testing thin‐film modules significantly contributes to the understanding of field performance and the reliability of emerging PV technologies. A number of programs are under way at NREL for testing flat‐plate PV modules and monitoring their performance in sunlight and under simulated conditions to provide accelerated performance characteristics that represent long‐term exposure to normal weather conditions. Modules are installed outdoors and connected to loads, and their performance is monitored. Other modules are mounted in environmental chambers and are exposed to the rigors of heating, freezing, and high humidity. Modules are exposed to simulated rain and are tested for electrical leakage and are also exposed to simulated hail stones to determine structural integrity. Performance tests are conducted under standard test conditions (STC) in accordance with ASTM E1036. This paper presents some of the details of PV module testing at NREL.