Three-dimensional (3-D) interactive modeling permits integrated processing and interpretation of gravity and magnetic data, yielding an improved geologic interpretation. 3-D model bodies are constructed from polyhedra of suitable geometry and physical parameters (density and susceptibility), input on an interactive graphics terminal that is tied to a host computer. The method is especially designed for concurrent processing and interpretation in an interactive mode. The effect on gravity of a homogeneous polyhedron is calculated by transforming a volume integral into a sum of line integrals. Magnetic effects can be modeled by using either Poisson’s theorem or a slight modification of the formulas derived for gravity modeling. The interactive modeling program allows the user to change the geometry as well as the density and/or susceptibility of the elementary polyhedra and to observe results quickly during the course of processing. This capability enables the interpreter to decide immediately if and where a tentative geologic structure must be changed for the modeled effect to fit that of a field survey. He is able to drive the device-dependent process by clear menu functions without any knowledge of the rather complicated data structure and the interaction between the main program and its many subroutines. In addition, application of this method requires considerably less computing time than conventional methods based on the direct evaluation of volume integrals.