MELSAR, a mesoscale air quality model, was developed for predicting air pollutant concentrations resulting from releases from multiple sources. The model is a Lagrangian puff model for application in complex terrain, principally at long source-to-receptor transport distances (tens to hundreds of kilometers) and short pollutant averaging times (1 to 24 hr). Terrain influences are treated explicitly by using a three-dimensional mass-consistent flow model, accounting for the influences of terrain roughness on diffusion, using terrain-following plume trajectories with optional corrections for the plume ascending hills, and using a parameterized treatment of pollutant sources located in valleys. The model handles releases from both point and area sources and makes the conservative assumption that the pollutants released are inert and nondepositing. MELSAR is developed for application in a region covering western Colorado, eastern Utah, and southern Wyoming.