The importance of the micro stress field in the failure of discontinuous rocks is emphasized. The factors controlling failure are identified as, the micro stress set up at the ends and at irregularities of joint segments, the macro stress which controls the growth of the microfracture to macroscopic dimensions, and the external constraint which may arrest the propagating failure surface. By causing microfracture, the micro stress is largely responsible for the destruction of the rock bridges and the asperities occurring along the plane of weakness. In the initial stages of failure, the various types of tensile and shear microfractures form a zig-zag pattern which on continued deformation becomes incorporated in a wide shear zone running along the original plane of weakness.