Nearly 95 per cent of geophysical expenditures for oil exploration is for reflection seismograph surveys. Most of the other 5 per cent is for gravity and magnetic surveys. To some extent the disproportionately large expenditures for seismograph work probably are from a lack of understanding and appreciation of the usefulness of the other methods. The paper reviews the basic principles of the gravity and magnetic methods, points out their similarities and differences, and gives examples of their application. These examples include samples of basement depth maps from magnetic surveys where independent control on actual depths was available. A comparison with this control indicates that depth determinations from magnetic surveys may have a reliability of approximately ± per cent. Regional gravity effects and their removal by graphical and grid calculation schemes are illustrated by examples from the Gulf Coast and California. Methods of quantitative interpretation of gravity are demonstrated by examples from the salt dome area of the Gulf Coast.