The public trust clause of Pennsylvania’s environmental rights amendment recognizes the commonwealth’s responsibility to “conserve and maintain” “public natural resources,” and states that these resources are “the common property of all the people,” including future generations. But what does it mean for these resources to be common property? This Article argues, first, that the term includes resources expressly recognized in well-developed common law as government-owned prior to the amendment, like state forest and park land, beds of navigable waters, and publicly dedicated land, as well as types of resources such as air, water, fish, and wildlife, that no one categorically owned in a “title-based” sense. Second, it uses long-standing public trust law to explain the commonwealth’s constitutional duties to manage these resources as common property. These duties involve public availability of these resources, which includes but is not limited to their protection. This understanding of Pennsylvania’s public trust clause provides a basis for strengthened protection of public natural resources, and is central to the meaning of the public trust doctrine in other states and countries, focusing the government’s trust duties on those resources most essential for human survival and well-being.