Only within the past 15–20 years have petroleum explorationists really begun to recognize the economic significance of deltas. With this recognition has come a realization of the variabilities of deltas both external and internal. For decades, little more was known about ancient deltas than that which was published by Barrell (1914) on the classic Catskill-Chemung delta of the Appalachian geosyncline and the much smaller deltas of glacial Lake Bonneville described by Gilbert (1885). In more recent years, several significant papers treating ancient deltas have appeared in the literature. Busch (1953) described the Pennsylvanian Booch delta of the Arkoma basin; Nanz (1954) the Oligocene sandstone reservoir of the Seeligson field of the Gulf Coast; Weimer (1961) the Late Cretaceous Rawlins delta; Halbouty and Barber (1961) an Oligocene delta of the Gulf Coast; Rainwater (1963) a Gulf Coast Miocene delta; and Swann (1964) a Late Mississippian Michigan River delta. In reviewing these papers, it becomes apparent that no two of the deltas described are alike and that considerable background relative to modern deltas and their respective depositional environments is an essential prerequisite to their recognition and delineation in the subsurface. Modern deltas have certain distinct differences, too, which are related to such variables as stream gradient, stream density (suspended and dissolved load), relative abundance and type of suspended load, basin-water density, width of the mouth of the effluent stream, shape of the depositional basin, and energy levels within the basin.