Shales comprise a large proportion of most sedimentary basins and form the seal and source rocks for many hydrocarbon reservoirs. They also have a major impact in gas plays involving shale where they act as both the source rock and reservoir. Because of their low permeability, there is great interest in using shales as host rocks for waste storage. In the petroleum industry, shales cause billions of dollars in losses annually through, for example, pore-pressure-related kicks, blow-outs, and wellbore instability. Shales have a decisive impact on fluid-flow and seismic-wave propagation because of their low permeability and anisotropic microstructure.