Abstract: The purposes of this work have been to obtain some general information on the actual mechanism of deformation and failure in both laboratory and `in-situ` shear test; and to shed light upon the development of progressive andretrogressive failure under varius test conditions.The study includes extensive laboratory testing and a theoretical approach to progressive deformation. In the latterapproach, a computer model of a shear block with certain discontinuities was developed. The method of calculation utilizes finite-element technique for the determination of the state of strain and stress in elastic and/or elastoplastic structures, for an assessment of the mechanical influence of a given discontinuity within the elastic structure, for a systematic evaluation of the effects of type of loading conditions on shear deformation and failure characteristics of the blockand to examine the actual mechanism of shear deformation and failure in a test block under direct shear loading. Thetheoretical program was accompanied by mechanisal laboratory tests designed to test the reliability and applicability ofconclusions suggested by the theoretical studies.It has been concluded that choice of boundarry conditions exerts an important control on failure mechanism. The tensile zones, which always develop within the test block, are of mechanical importance. Local failure which occurs in thesetensile zones, in the ‘separation’ mode, under certain boundary conditions, leads to progressive failure. Straight-linefailure envelopes can be interpreted as representation of a single (shear) failure mode. Parabolic failure envelopes aresuggestive of multiple modes of failure.‘In-situ’ shear tests on geological materials should be interpreted in more sophisticated terms; i. e., as a consequenceof variable combined stress state, involving inhomogeneous stress field; extensive stress reorientation, .and multiple crackpropagation.The theoretical approach to the mechanism of deformation and failure characteristics of `in-situ` shear tests, utilizingfinite-element method, appears to be a valid approach for prediction of certain experimental results.