Observations of brittle failure at the laboratory scale indicate that the brittle failure process involves the initiation, growth, and accumulation of micro-cracks. Around underground openings, observations have revealed that brittle failure is mainly a process of progressive slabbing resulting in a revised stable geometry that in many cases take the form of V-shaped notches. Continuum models with traditional failure criteria (e.g. Hoek-Brown or Mohr-Coulomb) based on the simultaneous mobilization of cohesive and frictional strength components have not been successful in predicting the extent and depth of brittle failure. This chapter presents a continuum modeling approach that captures an essential component of brittle rock mass failure, that is, cohesion weakening and frictional strengthening (CWFS) as functions of plastic strain. A Strain-dependent rock brittleness index is defined which can represent the mobilized strength during brittle failure of rock.