The field reserves evaluation strategy developed in this paper is a tool to help recognize and quantify the key uncertainties in field reserve estimates before committing to development. Reserve estimate changes through time for UKCS fields developed over the last 20 years are examined. We examine the causes of these changes and propose a scheme to assess fields in terms of structural complexity and reservoir quality and architecture using nine categories. Applying this scheme, termed "Geoscore", to 33 UKCS fields we show that reserve estimates: (a) for low complexity (low Geoscore) fields are underestimated by an average of 20%; (b) for medium complexity fields are overestimated by 10%; (c) for high complexity (high Geoscore) fields are overestimated by 15%. There is also a trend of decreasing recovery factor with increasing Geoscore. Accurate reserve estimation during field appraisal requires: recognizing the key uncertainties; quantifying the relative importance of these uncertainties; either collecting data or preparing contingency plans to handle potential problems that may arise during field development. We describe the key uncertainties, types of data to be collected and how to handle the remaining uncertainties. Careful application of the Geoscore analysis helps the recognition and quantification of these key uncertainties.