Peak Oil is the simplest label for the problem of energy resource depletion, or more specifically, the peak in global oil production. Oil is a finite, non-renewable resource, one that has powered phenomenal economic and population growth over the last century and a half. The rate of oil 'production,' meaning extraction and refining (currently about 84 million barrels/day), has grown in most years over the last century, but once we go through the halfway point of all reserves, production becomes ever more likely to decline, hence 'peak'. Peak Oil means not 'running out of oil', but 'running out of cheap oil'. For societies leveraged on ever increasing amounts of cheap oil, the consequences may be dire. Without significant successful cultural reform, economic and social decline seems inevitable.

In the 1950s a U.S. geologist working for Shell, M. King Hubbert, noticed that oil discoveries graphed over time, tended to follow a bell shape curve. He posited that the rate of oil production would follow a similar curve, now known as the Hubbert Curve (see figure). In 1956 Hubbert predicted that production from the US lower 48 states would peak between 1965 and 1970. Shell tried to pressure Hubbert into not making his projections public, but the notoriously stubborn Hubbert went ahead and released them. In anycase, most people inside and outside the industry quickly dismissed Hubbert's predictions. It turned out that Hubbert was right, US continental oil production did peak in 1970.