Geologic fracture mechanics (GFM) can be thought of as an interdisciplinary field combining approaches from engineering, materials science, and geology. It includes Linear Elastic Fracture Mechanics (LEFM) but relaxes some of the assumptions that are required for LEFM to apply to geologic structural discontinuities (i.e., fractures and deformation bands). LEFM is widely regarded as the most simple and restrictive special case of fracture mechanics (see discussions by Latzko, 1979; Kanninen and Popelar, 1985, p. 13; and Anderson, 1995, p. 117). Upon close examination, it may be seen that many of the predictions of LEFM do not match geologic observations as well as might be desired, suggesting the need for a more general approach that includes material from chemistry (to better consider diagenesis (Fig. 9.1) and subcritical fracture propagation) and plasticity (to better represent near-tip processes). Elements of some of these approaches are described in this chapter.