Muds and mudstones are the prime control on fluid flow in sedimentary basins and near surface environments. As the world's most common sediment type, they act as aquitards in sedimentary basins, restricting water flow and influencing the development of overpressure. In petroleum systems they act as source rocks for nearly all oil and much gas, determine migration directions between source and trap in most settings, and act as seals to many reservoirs. In near surface environments they not only control natural flow but have been commonly used to restrict leakage from waste disposal sites. This volume focuses on the poorly described physical properties of muds and mudstones in both near-surface and deep basinal settings. Amongst its contents are reviews of compaction, permeability, thermal conductivity and mechanical properties of mudstones, experimental studies of transport properties and case studies of the importance of fluid flow in both hydrogeological and petroleum systems