This chapter provides a synopsis of the use of 2-D (two-dimensional) stress in rock deformation. First, we’ll look at how to apply these simple equations to problems in Coulomb frictional sliding along surfaces in rock. Then we’ll introduce two other very useful, but somewhat more involved, failure criteria for rocks. By the end of the chapter you should be able to start with the stresses on a small piece of intact rock, know how to deal with either a simple Coulomb slip surface or a crack, and then apply this to understand the field-scale characteristics of large-scale fractured outcrops (Fig. 3.1).