A review is given of recent fluid modeling studies conducted at the EPA Fluid Modeling Facility of flow and diffusion in complex terrain. Estimates are given for the ratio of the maximum concentration on a hill surface to the maximum concentration in the absence of the hill. The ratio may be regarded as a terrain correction factor and is a function of hill aspect ratio (two-versus three-dimensional), hill slope, atmospheric stability, etc. For upwind sources, terrain correction factors are typically 1 to 2 for neutral flow over three-dimensional hills. Terrain correction factors as large as 10 to 15 were found for low sources placed downwind of two-dimensional hills of moderate to large slope. For strongly stable flow over three-dimensional hills, it is more useful to compare maximum surface concentrations with those at the centerline of the plume in the absence of the hill. These concentrations have been shown to be essentially equal.