Mudrock is the most abundant material in the uppermost 5 km of the Earth’s crust (Petley, 1999). It has low permeability and undergoes enormous compaction during burial. When mudrock is loaded sufficiently rapidly (e.g., by burial or tectonic loading), the load is borne by both the solid grains (as effective stress) and the pore fluid (as overpressure) because the fluid cannot escape at the rate the loading occurs. The compressibility of the rock defines how much fluid will be expelled, and its permeability defines how rapidly that fluid can be expelled. Because of mudrock’s high compressibility and low permeability, these overpressures can be maintained for geological timescales. Thus, the compaction behavior of mudrocks is one of the most important controls on whether overpressures will form. The degree of compaction is a sensitive indicator of the effective stress. I show in Chapters 5 and 6 how the compaction state is used to interpret the effective stress and ultimately the pore pressure.