One objective of the U.S. Environmental Protection Agency's (EPA's) Supersite Program was to provide data that could be used to more thoroughly evaluate and improve air quality models, and then have those models used to address both scientific and policy-related issues dealing with air quality management. In this direction, modeling studies have used Supersites-related data and are reviewed here. Fine temporal resolution data have been used both to test model components (e.g., the inorganic thermodynamic routines) and air quality modeling systems (in particular, Community Multiscale Air Quality [CMAQ] and Comprehensive Air Quality Model with extensions [CAMx] applications). Such evaluations suggest that the inorganic thermodynamic approaches being used are accurate, as well as the description of sulfate production, although there are significant uncertainties in production of nitric acid, biogenic and ammonia emissions, secondary organic aerosol formation, and the ability to follow the formation and evolution of ultrafine particles. Model applications have investigated how PM levels will respond to various emissions controls, suggesting that nitrate will replace some of the reductions in sulfate particulate matter (PM), although the replacement is small in the summer. Although not part of the Supersite program, modeling being conducted by EPA, regional planning organizations, and states for policy purposes has benefited from the detailed data collected, and the PM models have advanced by their more widespread use.