Fracture and flow of rocks under stress and their geophysical and seismological implications raise fundamental questions in rock mechanics, particularly in the areas of tectonophysics and seismology. This text exclusively addresses the deformation and fracture of rock specimens under general triaxial compression, in which all three principal stresses are different. This effect has often been disregarded by other strength theories, but current research reveals that it significantly affects the ultimate strength of rocks. Intermediate principal stress strongly influences the deformation and failure mode of rocks, and is described here in graphic and numerical form. The effect of the intrinsic features of rock masses of inhomogeneous or anisotropic structure are taken into account, as are acoustic emission phenomena in rocks under various stress states. Friction in rocks, measured by a newly-designed shear-testing machine, is discussed in relation to earthquake phenomena.