Models3 is EPA's third generation airquality modeling system. A computer-based modeling framework, Models-3, has been developed to simplify the use and continued enhancement of environmental models. The software contains a Community Multi-scale Air Quality (CMAQ) modeling system for urban to regional scale air quality simulation of tropospheric ozone, acid deposition, visibility, and fine particles. Models-3 and CMAQ in combination form a powerful third generation air quality modeling and assessment system that enables users to execute air quality simulation models for their specific problem domain and visualize the results. Models-3/CMAQ also assists the model developer to assemble, test, and evaluate science process components and their impact on chemistry-transport model predictions by facilitating the interchange of science codes and the modification of key model specifications such as grid resolution and chemistry mechanism without modifying the code. The primary goals for the system are to improve: 1) the environmental management community's ability to evaluate the impact of air quality management practices for multiple pollutants at multiple scales; and 2) the scientist's ability to better probe, understand, and simulate chemical and physical interactions in the atmosphere. Another goal of the Models-3 program is to make complex and advanced models more available to everyday users, especially those people in state and local governments, industry, and academia. The Models-3 release contains two types of environmental modeling systems: meteorological, emission, and chemistry transport. It also includes a visualization and analysis system.