The seismic velocity structure of the Earth’s crust is examined in relation to the role played by high pore pressures. Compressional and shear-wave velocities measured at carefully controlled confining and pore pressures show significant decreases with increasing pore pressure. This behavior is important in sedimentary rocks and in crystalline rocks likely to occur at deep crustal levels. Velocities are shown to be a function of effective pressure rather than differential pressure. Similar results are found for crystalline rocks in which gas or water is the pore fluid. Thus, pervasive CO2 at elevated pore pressures will lower crustal velocities. Low-velocity regions originating from high pore pressures have high Poisson’s ratios. Within the crust, dehydration accompanying progressive metamorphism can produce high pore pressures and regions of low velocities if pore fluids are contained. A conceptual model is presented for the continental crust in which midcrustal discontinuities such as the Conrad, where present, separate an over-pressured upper region of igneous and metamorphic rocks from underlying dry rocks. Fluids released by lower crustal dehydration are trapped above the Conrad discontinuity. In midcrustal regions, where pore pressures are low due to loss of fluids, progressive metamorphism with depth will be detected as a gradual increase in velocity rather than a sharp discontinuity.