Models-3 and Community Multi-Scale Air Quality (CMAQ) software 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 specification of key model features 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 people in state and local governments, industry, and academia. The Models-3 release uses two types of environmental modeling systems, emission, and chemistry transport, and a visualization and analysis system.