The flow structure over a bell shaped hill (reciprocal of a fourth order polynomial in cross section and height h) was studied in large and small stably stratified towing tanks (with uniform density gradients) and in an unstratified wind tunnel. Observations were made at Froude numbers F=U/(Nh) over the range 0.1 to 1.7 and at F=infinity (U is the towing speed and N is the Brunt-Vaisala frequency). For F greater than or equal to 0.4, the observations verify Drazins' (1961) theory for low Froude number flow over 3-dimensional obstacles and establish limits of applicability. For Froude numbers of order 1, the study shows that a classification of the lee wave patterns and separated flow regions observed in 2-dimensional flows also appears to apply to 3-dimensional hills. Flow visualization techniques were used extensively to produce both qualitative and quantitative information on the flow structure around the hill. Representative photographs of dye tracers, potassium permanganate dye streaks, shadowgraphs, surface dye smears, and hydrogen bubble patterns are included. While emphasis is centered on obtaining a basic understanding of flow around 3-dimensional hills, the results are applicable to estimating air pollutant dispersion around hills.