Rock Nature. Geology. Mineralogy. Petrology, rock types. References. Rock Structure. Origin and nature of crystalline rock structure. Scale factors. Categorization. Structural modeling. Identification, determination and recording of structural features. Structural homogeneity. Example of structural characterization. Rock Stresses. Introduction. The concept of principal stresses. Stress conditions. Groundwater (piezometric) pressures. References. Rock Strain. Introduction. Definitions. Viscous and plastic strain. Calculation of strain in rock mechanics. Examples. References. Rock Strength. Introduction. The strength of rock material. The strength of rock masses. What strength criteria shall we use?. Material modelling. Determination of rock strength. References. Rock Stability. The meaning of rock stability. Tectonically and thermally induced strain. Rock displacements by loading or unloading. Rock fall. Clay-weathered rock and clayey sedimentary rock. Examples. References. Rock Excavation. Introduction. Rock Support. General principles. Rock Classification. General aspects. The Q-index. Practical use of Q-data in ordinary projects. Application of the Q-system to HLW repositories. Conclusions. References.