Water temperature fundamentally influences aquatic diversity and ecosystem health. In Colorado, temperature water quality criteria were revised in January 2007 based on a rigorous evaluation of the thermal requirements of fish species resident in Colorado. This article presents an account of how this process was conducted, and details the resultant criteria. The purpose of developing these criteria was to protect coldwater and warmwater fishes, especially native species such as cutthroat trout (Oncorhynchus clarki), from thermal stress. As such, lethal temperatures and optimal temperature conditions were determined from a literature review for species of the state, and these data were compiled into the Colorado Temperature Database. Acute and chronic thermal thresholds were then calculated for individual fish species. Finally, assemblages of fish were grouped into thermal tiers and temperature criteria were developed based on biological criteria for each assemblage. A case study is presented detailing the integration of science and policy decisions that shaped the development of Colorado's coldwater temperature criteria. Some issues were not resolved during this revision of Colorado's temperature water quality criteria, including protection from thermal shock and from gross changes in diel and seasonal thermal variability.