Seals are an important and commonly overlooked component in the evaluation of a potential hydrocarbon accumulation. Effective seals for hydrocarbon accumulations are typically thick, laterally continuous, ductile rocks with high capillary entry pressures. Seals need to be evaluated at two differing scales: a “micro”scale and a “mega” or prospect scale. Quantitative “micro” data measured on hand specimens of seal rock are difficult to extrapolate a billion-fold to the scale of the sealing surface for a hydrocarbon accumulation. Fortunately, each class of exploration prospects has a different set of seal problems. Geologic work can be focused on the characteristic seal problems that plague classes of prospects. Anticlines have relatively little seal risk, because any layer serving as a top seal will also be a lateral seal. Stratigraphic traps and faulted prospects have substantial seal risks. When attention is focused toward the expected sealing surface of a potential hydrocarbon accumulation, it is possible to assess the relative risk that a seal is present. Improvements in assessing seal risk for an exploration prospect directly affect the estimation of exploration success.