This survey summarizes what is known about the magnetic field of the earth, based on information available through 1971. Observed phenomena are described and interpreted in terms of the most widely accepted physical explanations. An overview of the geomagnetic field and its dynamic relationship with its terrestrial and interplanetary environment is first presented as briefly as possible, followed by more detailed descriptions of geomagnetic measurements and experimental methods, precise current models and past behavior of the main field, regular variations in the field resulting from the motion of the earth, disturbance variations of the field produced by interplanetary environment, and dynamic processes occurring in the outer magnetosphere. Extensive references is a replacement for Chapter 11 of Handbook of Geophysics and Space Environment (Shea L. Valley, ed.), Air Force Cambridge Research Laboratories and McGraw-Hill Book Co., New York, 1965.