Substantial public budget constraints across the United States have increased reliance on charities for some public service provision. This article builds a unique historical dataset and presents the first systematic look at the charities supporting U.S. national and state parks through the lens of coproduction along with other major theories of government-voluntary interaction and its consequences. The results suggest that parks-supporting charities are involved in a multi-dimensional pattern of coproduction with distinct and differentiated forms of involvement in public service provision at the state and federal levels. Their growth over time reflects theories of government failure and philanthropic insufficiency. And their permanency suggests the value of greater understanding of public service reliance on private philanthropy.