The contributions of the EPA Fluid Modeling Facility (FMF) to the Complex Terrain Model Development Program (CTMDP) are described. These contributions included a wide range of laboratory studies and a limited amount of numerical modeling of flow and diffusion in neutral and stably stratified conditions in complex terrain. The goal of the CTMDP is the development of a dispersion model valid in complex terrain, with emphasis on plume impaction on nearby hills during nighttime stable conditions. Work at the FMF prior to the inception of the programs provided the basic framework for the model - the dividing-streamline concept - and the focal point around which the field program was designed. Throughout the course of the CTMDP, the FMF interacted vigorously with the model developers by providing support in various ways. Simultaneously, the FMF responded to the needs of the EPA Office of Air Quality Planning and Standards (OAQPS) by providing guidance concerning expected terrain effects and by conducting demonstration studies. Finally, several supplemental studies were conducted, broadening and expanding upon the specific requests of the model developers and the OAQPS.