The search for a simple, universal classification of petroleum reserves has become a sort of mythical quest whose importance grew while the international petroleum industry developed. Reserves have always been a fundamental element of the industry, and the need for a uniform set of definitions has been a continuum not only for the operators but also for all the public and private organizations which work in related areas. The definitions recently adopted by the Society of Petroleum Engineers and the World Petroleum Congresses represent a step forward in this essential issue. This paper summarises the current definitions, and introduces two significant changes: the use of probabilistic methods in reserves,es assessment; and the provision of an "averaging" period in the determination of applicable economic conditions The latter seems particularly appropriate for a year such as 1998 considering the recent fall lin oil prices.