The high incidence of oil in the Big Horn Basin of Wyoming is primarily the result of the position of the basin on the stable foreland shelf of the deposition basin between the Cordilleran geosyncline and the interior basins throughout Permian, Pennsylvanian, and Mississippian time. A subsidiary deposition basin which formed on the site of the present basin during Permian time contributed largely to the occurrence of oil in Phosphoria and Tensleep rocks. Later structural deformation which formed closed anticlines further localized the oil. The present occurrence of Paleozoic oil in the basin is dependent upon the age of structure, the relation of anticlines to the subsidiary Permian basin, the development of porosity, and the effect of water circulation. Additions to the known reserves of this basin will be discovered through proper understanding and application of these controlling factors of oil occurrence.