Strain localization is a phenomenon closely linked to rupture in geomaterials. Experimental and theoretical studies of strain localization has been a very active research subject in the last 30 years, first for soils then for rocks and, more recently, concrete. Presently, the new approaches that resulted from the research enter progressively in the common background of advanced geomechanical engineering. The course exposes the basics of the experimental and theoretical knowledge about strain localization in geomaterials. A detailed description of the experimental features is presented for granular bodies like sand, and for cohesive bodies like rocks. The results discussed are often illustrated by observations coming from the author’s experimental work, however a broader view is given by incorporating other authors results as well. On the theoretical side, the analysis of the onset of strain localization using shear band analysis is presented, and illustrated for different kind of constitutive equations.