The cataclasites presented in this chapter are all cohesive fault rocks that show evidence of brittle fracturing although other processes such as grain boundary sliding and pressure solution may have played a role in their formation as well. These rocks are usually composed of angular broken rock fragments embedded in a matrix of quartz, iron oxide, calcite, chlorite and/or other minerals that precipitated from a fluid. Cohesive cataclasites are thought to form in the P-T realm where brittle deformation predominates, that is, approximately in the upper 10 km of the Earths crust, with lithostatic pressure up to about 3 kbar and temperatures up to about 300 °C (Fig. 1.1). However, this depth cannot be established with precision because other factors, like the presence or absence of fluids and the strain rate, play also an important role.