The physical foundations and justification of plasticity theory for application to rock and soil, geo-materials, is necessarily based on experimental evidence. For many years conventional wisdom indicated rock was a brittle material that failed violently by fast fracturing at the elastic limit. Plasticity theory was for ductile materials that showed large post-yield strain. This view was strongly conditioned by laboratory testing of intact rock cylinders that, indeed, often shattered at failure with an audible bang. Of course, no further plotting of a force-displacement (stress-strain) graph was possible; the “story” of the sample response to load was ended. However, the amount of plastic strain that occurs after reaching the elastic limit is not a requirement of plasticity theory as such. Moreover, rock masses in nature are almost always fractured, “jointed,” and certainly the story of the rock mass response to load is not ended. Additional elastic deformation may be possible in response to further loading, perhaps induced by excavation. Deformation beyond the range of a purely elastic response is also possible and indeed may even be likely. A plasticity theory is needed to describe deformation beyond the elastic limit and thus to continue the “story.”