It has long been known that the Coulomb-Mohr criterion does not satisfactorily account for the direction of faults and fractures in the Earth. Modern numerical methods now permit the use of a better criterion, i.e., that fracture initiates and propagates in the direction in which the strain energy release is maximum for a given crack length. This criterion is thermodynamically correct, can be used for heterogeneous and elasto-plastic bodies, and yields results in good agreement with laboratory data, but it is somewhat computer intensive. ¿ American Geophysical Union 1992