Stable crack growth strongly influences both the fracture strength of brittle rocks and some of the phenomena precursory to catastrophic failure. Quantification of the time and environment dependence of fracture propagation is attempted with the use of a fracture mechanics technique. We examine some of the difficulties encountered when applying techniques originally developed for simple synthetic materials to complex materials like rocks. A picture of subcritical fracture propagation is developed that embraces the essential ingredients of the microstructures, a microcrack process zone, and the different roles that the environment plays. To do this we examine the results of (1) fracture mechanics experiments on five rock types, (2) optical and scanning electron miccroscopy, (3) studies of microstructural aspects of fracture in ceramics, and (4) exploratory tests examining the time-dependent response of rock to the application of water.