Simple calculations, assuming vertical flow and no lateral drainage within the basin, show that overpressures approaching fracture pressure in the overburden will be reached if the effective permeability of the shale forming the seal is less than 0.1–0.01 nD (1.10−22–1.10−23 m2). The permeability of shales varies greatly as a function of primary textural and minnerlogical composition and it is not possible to accurately predict the effective permeability of a sequence of shales forming pressure barriers. Overpressure in uplifted basins, where there is no compaction taking place, can only be maintained over geological time if the permeabilities are much lower. Overpressure is often controlled by lateral drainage but the effective permeabilities for fluid flow across faults and the offset of permeable layers are also difficult to predict. In most cases, uncalibrated basin modelling is unable to accurately predict the magnitude and distribution of overpressures because the vertical and horizontal permeabilities in sedimentary basins cannot be determined in sufficient detail. In basins that have been extensively explored and developed, incorporation of prior geological knowledge into basin models may allow overpressures to be predicted ahead of the drill bit. However, with such a large body of information already gathered about the basin it is debatable what extra value basin modelling is providing to pressure predictions.