The question of uniqueness or otherwise of the various geophysical methods has been looked into from a general point of view. A method is theoretically determinate or indeterminate according as the totality of unknowns is smaller or greater than that of the independent measurables. While all natural field methods fall in the latter category, all applied field methods do not necessarily come under the former. Actual interpretations, however, cannot make any use of the theoretical uniqueness even where it exists and must depend on simplifying but permissible hypotheses based on extraneous information. These hypotheses are found to be surprisingly similar in all geophysical methods.