Abnormal pore pressures are encountered worldwide, often resulting in drilling problems such as kicks, blow-outs, borehole instability, stuck pipe, and lost circulation. Because of this, a quantitative predrill prediction of pore pressure is required for the safe and economic drilling of wells in overpressured formations. In this paper, the basic concepts used in pore-pressure prediction are defined, and the way in which pore pressure can be estimated using velocity measurements is explained. A predrill estimate of pore pressure can be obtained from seismic velocities using a velocity-to-pore-pressure transform calibrated with offset well data. However, velocities obtained from processing seismic reflection data often lack the spatial resolution needed for accurate pore-pressure prediction, due to assumptions such as layered media and hyperbolic moveout. In the following paper in this section, Chopra and Huffman review the available methods of velocity model building and discuss the advantages and disadvantages of each. Once suitable velocities are obtained, a velocity-to-pore-pressure transform is required. This article provides the rock physics basis underlying such transforms.