An endeavor is made to outline the history of that phase of the conservation movement which has led to unit operation since the World War period; also to show that the geologist has played a vital part in furthering its cause and in bringing the petroleum industry to an appreciation of the advantages of developing an oil and gas structure as a geological unit.

A summary is given of the geology of six oil and gas fields involving public lands in the western United States now under unit operation, namely, North and Middle domes of Kettleman Hills, California, Little Buffalo Basin, Big Sand Draw, Billy Creek, and Pitchfork fields in Wyoming. The geology of the McCallum field, North Park, Colorado, for which unit operation is contemplated, is also reviewed.

The Federal laws and regulations governing unit operation on lands in which mineral title is still in the Government are cited and the salient geological and administrative features to be considered in the presentation of plans for unit operation of cooperative development for approval by the Secretary of the Interior are suggested.