Oblique slip faulting could develop under the influence of an oblique stress system or as a result of inhomogeneity of strength or elastic properties. The dynamics of fracture within a preferred plane is studied, and the direction of movement immediately after fracture has been found to coincide with the direction of maximum shearing stress within the fracture plane. The direction of this stress within a given plane subjected to a general stress system has been derived. For a stress system of given orientation it is found that the initial slip may occur in any possible direction within the plane, the direction depending on therelative values of the three principal pressures. Theoretically, if a preexisting fault is subjected to a reoriented stress system, oblique movement will occur after fracture.