Subsurface or field appraisal is an activity aimed at reducing reservoir uncertainty. For optimal results from appraisal, it is necessary to have a clear understanding of appraisal, to distinguish between two types of appraisal, and to use a decision tool that ties reservoir uncertainty to potential economic consequences in the event of development. The tool proposed is "value of information" (VOI), which provides a consistent, auditable criterion for decision-making. In the case of optimization appraisal, VOI enables an analysis of the economic merits of a series of development scenarios with and without appraisal information. The probability distribution of the underlying uncertainty, e.g. reserves, is a key ingredient. A truly cross-disciplinary effort is called for. The ultimate goal in appraisal is to reduce economic risk with regard to field development.