His book The Ultimate Resource, later reissued as The Ultimate Resource 2, is a massive assault on the conventional wisdom of population growth and resource consumption. In it, Simon challenged the notion of a pending Malthusian catastrophe - that an increase in population has negative economic consequences, that population is a drain on natural resources, and that we stand at risk of running out of resources through over-consumption. His critique was praised by Nobel Laureate economist Friedrich Hayek.
In 1980 Julian Simon and Paul Ehrlich entered into a famous wager, betting on a mutually agreed upon metric of resource scarcity. Simon won the bet. He proposed that they renew the bet since Ehrlich continued to claim that the price of metals would rise. Ehrlich refused. Simon proposed that they bet on a metric for human welfare. Ehrlich offered Simon a set of 15 metrics over 10 years, victor to be determined by scientists chosen by the president of the NAS in 2005. There was no meeting of the minds, because Simon felt that too many of the metrics measured attributes of the world not directly related to human welfare, e.g. the amount of nitrous oxide in the atmosphere.  
Simon was one of the founders of free-market environmentalism. An article profiling Julian Simon in Wired magazine inspired Bjørn Lomborg to write the revisionist environmental book The Skeptical Environmentalist.
Simon was also the first to suggest that airlines should provide rewards for travelers to give up their seats on overbooked airlines rather than arbitrarily keep certain passengers off the plane.
Simon was an omnivorous reader, and took some steps toward writing a memoir. He died at the age of 66.