The magnetic fields of the terrestrial planets are not only interesting in their own right, but are of general geological interest because among other reasons their existence may place important constraints on planetary thermal states and hence evolution. It is now generally accepted that planetary magnetic fields are generated by dynamo action resulting from convection in a rotating, electrically conducting fluid core. However, major uncertainties still exist in our understanding of such processes some 30 years after the core dynamo concept was introduced by E. C. Bullard, W. M. Elsasser, and others. The surprising discovery of lunar magnetism as a result of the Apollo program and the growing body of data which show high paleofield values in meteorites have raised the possibility that some planetary magnetic fields may be due primarily to permanent magnetization acquired early in the body's development. The need for an objective assessment of all the contending sources of planetary magnetism and the need for communication among the highly specialized and diverse groups working in this area resulted in the organization of a topical conference entitled ‘Origins of Planetary Magnetism,’ held at the Lunar and Planetary Institute, Houston, Texas, on November 8–11, 1978. Seventy-four participants from the U.S. and five foreign countries met for 3½ days to promote the assimilation and comparison of the data available on planetary magnetic fields and to examine these data in the light of our knowledge of the earth's field, recent advances in dynamo and other theories, and our knowledge of planetary interiors and their thermal and chemical evolution.