This paper examines the factors responsible for changes in crude oil prices. The paper reviews the statistical behavior of oil prices, relates these to the predictions of theory, and looks in detail at key features of petroleum demand and supply. Topics discussed include the role of commodity speculation, OPEC, and resource depletion. The paper concludes that although scarcity rent made a negligible contribution to the price of oil in 1997, it may be an important feature of the most recent data.