The U.S. Environmental Protection Agency (EPA) is sponsoring the Complex Terrain Model Development project, a multi-year study to develop improved models for calculating ground-level air pollutant concentrations that result from large emission sources located in mountainous terrain. The project started with tracer and flow visualization experiments and fluid modeling for isolated, simple terrain features--Cinder Cone Butte (CCB), Idaho and the Hogback Ridge (HBR), New Mexico--and has progressed to the Full Scale Plume Study (FSPS), conducted at the Tracy Power Plant near Reno, Nevada during August 1984. The CCB and HBR experiments and related fluid modeling have demonstrated the usefulness of the concept of a dividing-streamline height (Hs) in discriminating between essentially horizontal flow that goes around the sides of the terrain features and flow that goes up and over the terrain. An analysis of tracer gas concentrations observed at CCB and HBR showed that the highest concentrations occurred during direct impingement of plumes released at elevations near the height of the dividing streamline.