We review the methods used to estimate subsurface risk and volumetric uncertainty over the exploration and production lifecycle. Whilst probabilistic techniques are essential to decision-making, we show they have limitations which must be understood. In frontier situations they are hard to justify and non-technical factors weigh heavily in decisions. As more information becomes available, probabilistic estimates are more justifiable, but results can be very sensitive to small changes in input parameters and to other factors. The nature of the transition to development varies. In simple accumulations, once an economic volume has been discovered, exploration risk is replaced by development execution risk. In the case of extensive reservoir complexes, the risk of not finding an economic volume may persist. In both situations, uncertainties relating to reservoir detail, fluid properties, and natural drive mechanisms become critical. Production data add to the complexity, as do interactions with external factors such as surface facilities. Resolving uncertainty at this stage, therefore, requires techniques to integrate and interpret large amounts of data. Given the subjective nature of much of the input into evaluations, likely human biases must be recognized.