Wind-tunnel and towing-tank studies conducted over the past 10 years at the U.S. Environmental Protection Agency's Fluid Modeling Facility (FMF) of flow and diffusion in complex terrain are reviewed. A primary impetus for the work was EPA's Complex Terrain Model Development Program (CTMDP)--designed to develop reliable atmospheric dispersion models applicable to large pollutant sources in complex terrain, with primary emphasis on plume impaction during nighttime stable conditions. The FMF interacted closely with model developers participating in the CTMDP and provided support in various ways through the conduct of a wide range of laboratory studies. At the beginning of the program, the FMF provided direct support as an aid to planning the details and strategies of the field experiments and testing the limits of applicability of the dividing-streamline concept. Later work included exercises of 'filling in the gaps' in the field data, furthering the understanding of the physical mechanisms important to plume impaction in complex terrain and in stably stratified flows in general, testing various modeling assumptions, providing data for 'calibration' of various modeling parameters, and testing the ability of the laboratory models to simulate full-scale conditions. Simultaneously, the FMF responded to the needs of the regulatory arm of EPA, the Office of Air Quality Planning and Standards (OAQPS), by providing guidance concerning expected terrain effects and by conducting demonstration studies. These latter studies were concerned primarily with simulation of diffusion in the neutral atmospheric boundary layer. Finally, several supplemental studies were conducted, broadening and expanding upon the specific requests of the model developers and the OAQPS. The highlights of the FMF complex-terrain research work are described herein.