The safe drilling of wells for hydrocarbon exploration or production requires that the wellbore pressure be maintained between the formation pore pressure and the maximum pressure the formation can withstand without fracturing. This task is made more difficult when drilling in overpressured areas, which require a quantitative predrill prediction of pore pressure with uncertainty to be made. A predrill estimate of pore pressure can be obtained from seismic velocities using a velocity-to-pore-pressure transform calibrated by offset well data. However, the uncertainty in velocity is often not quantified. This example from the Gulf of Mexico uses seismic velocities with uncertainty, derived by combining seismic velocities with well data (den Boer et al., 2006). Parameters in the velocity-to-pore-pressure transform are estimated using seismic velocities, density logs, pressure information, and well velocities derived by inverting time depth pairs from check shots in the area. Pore pressure and associated uncertainty are then predicted by sampling the region of parameter space consistent with available well data.