This paper explores the economic effects of natural and historical conservation in the US National Park System, the largest and oldest national conservation entity in the world. We assemble a detailed dataset on the history of the system and analyze its consequences in an event study framework. The results show that national park designations, park expansions, and the establishment of new parks all increase overall employment and income in the local economy. The magnitudes of some of these effects are considerably larger than economic contribution estimates used by the National Park Service. We study how impacts vary over time and across industries, and show spillovers that extend beyond tourism. We also investigate the potential negative externalities arising from parks due to traffic, air pollution, and the shifting of resource extraction to other public lands. Our findings have implications for conservation policy in the US and elsewhere.