The Geological Survey of Canada has conducted petroleum resource assessments of Canadian sedimentary basins to respond to a need for information concerning the extent of Canada's energy endowment. The evolution of these activities and methods, which began in the 1970's and continue to the present, is discussed in this paper. The first assessment of Canadian basins was conducted in the 1970's using a volumetric yield method, whereby the volume of sedimentary rock in a basin was multiplied by a hydrocarbon yield per unit volume factor. Later, a Monte Carlo approach was used. It required a knowledge of the exploration plays in a given basin and made use of a variety of pool parameters expressed as cumulative probability distributions. The Monte Carlo method did not account for the biased data-set problem that came from using a selective exploration process. A third assessment method was based on geochemical data and was used to estimate the amount of hydrocarbon generated and the expulsion efficiencies. The results from each of the three methods defied detailed economic analysis.Advanced statistical methods were gradually developed in the 1980's. By the end of this period, PETRIMES (the Petroleum Exploration and Resource Evaluations System) was developed. This system evaluates hydrocarbon potential by means of an exploration play definition coupled with compiled play data used to estimate undiscovered pool sizes. During this period, discovery process models were developed to account for the biased data. Estimations of individual pool sizes in the play were made and displayed graphically so that undiscovered pools could be identified in a statistically derived population of pools. Summed quantities of petroleum in undiscovered pools were used to define remaining expected play potential.The estimated undiscovered individual pool sizes inferred from assessments serve as direct input to economic analyses that examine which pools are viable prospects under specific economic conditions. This knowledge is useful to governments formulating energy policies and to petroleum companies setting exploration priorities.