The Complex Terrain Dispersion Model (CTDM) is a refined air quality model for use in complex terrain applications. Its use of meteorological input data and terrain information is different than current EPA models; considerable detail for both types of input data are required and are supplied by preprocessors specifically designed for CTDM. CTDM requires the parameterization of individual hill shapes using the terrain preprocessor and the association of each model receptor with a particular hill. A central feature of CTDM is its use of a critical dividing-streamline height (Hc) to separate the flow in the vicinity of a hill into two separate layers; one of which flows over the hill, the other around it. The model will calculate on an hourly basis how the plume trajectory and shape are deformed by each hill. The computed concentration at each receptor is then derived from the receptor position on the hill and the resultant plume position and shape. The User's Guide describes the computer program, and gives instructions on its use.