The influence of porosity reduction processes on the hydrologic characteristics of the earth's crust is examined. We present a simple mathematical model that shows the effect of porosity reduction on pore presque development in the crust. Pore pressure in excess of hydrostatic can be generated if porosity reduction rates are sufficiently high. Elevated pore pressure could in turn affect properties such as strength and seismic reflectivity. Indirect evidence from laboratory, theoretical, and field studies indicates that porosity reduction processes are active, or very likely to be so, in large portions of the crust. Porosity, permeability, and pore pressure in the earth's crust may well be time-dependent, even in regions not subjected to large deviatoric stresses.