This EPCA Phase III Inventory includes, for the first time, the entire onshore United States. This is a detailed review of Federal oil and gas resources and constraints on their development within 18 geological provinces. In addition, the rest of the country was extrapolated from the results of these provinces studied in detail. For the Federal agencies that manage public land (principally the Bureau of Land Management and the Forest Service) and the citizens they serve, this Inventory will serve primarily as a planning tool. It provides public land managers with additional information to help develop land management plans. This report is a critical step in evaluating whether the documented impediments and restrictions are appropriate, and to what extent they constrain oil and gas development. The Inventory examines in detail six geological provinces in addition to the twelve included in the Phase II of EPCA. These six provinces are Central Alaska (Yukon Flats portion); Southern Alaska; Eastern Oregon-Washington; the Ventura Basin in California; the Eastern Great Basin in Idaho, Nevada, Utah and Arizona; and the Williston Basin in Montana, North Dakota and South Dakota.