We report on a systematic study of the factors affecting the degradation of prototype electrochromic (EC) windows. Our research activities include accelerated lifetime testing, activation analysis studies (AAS), and interfacial analysis studies. EC windows made by several different US companies were subjected to alternate coloring and bleaching voltage cycles while exposed to simulated 1.2-sun UV irradiance in a temperature-controlled environmental chamber. The samples inside the chamber were tested under a matrix of four different environmental test conditions at nominal sample temperatures of 85°C: cycling under irradiance, cycling with no irradiance, irradiance with no cycling, and simple thermal exposure with no cycling. In another accelerated environmental test, we evaluated device performance as a function of activation spectra. We used a series of cut-on filters for performing AAS of photolytically induced degradation of EC window devices. Finally, to identify the interface at which degradation occurs during cycling, we employed electrochemical impedance spectroscopy (ECIS). The ECIS and electro-optical measurements were systematically and periodically repeated to discover any changes in electrochemical behavior as a function of cycling.