Inconsistency in approaches used for booking reserves, whether for internal or external reporting requirements, represents a significant problem for the industry. Despite all the efforts made to establish a broadly acceptable set of reserve definitions and the guidelines published by various regulatory bodies, many areas exist where different interpretations are adopted. Therefore, the key requirement of any system, consistency, is not achieved. Inconsistency occurs at two fundamental levels : (1) in the interpretation of what is meant by "reserves," particularly with respect to commerciality, and (2) about what volumes should be booked in specific circumstances (e.g., under production-sharing contract terms, where nonhydrocarbons are present, and where hydrocarbons are used on site for fuel). The first problem may be addressed by an integrated resource-reporting system that ensures that reserves are seen in their correct context as discovered, commercial, recoverable, remaining resources. While the new Society of Petroleum Engineers/World Petroleum Congress (SPE/WPC) definitions provide a basis for the system, additional definitions and guidelines are required to ensure that an acceptable level of consistency is achieved. The second area of difficulty is related to the actual volumes booked. This is an issue that has not been addressed adequately to date. We propose a series of "guiding principles" as the basis for discussion and debate in the hope that it may lead to an improved understanding of the problem and, ultimately, to much greater consistency in the booking of reserves.