Ecosystems (natural capital) produce a range of benefits to humans. Natural capital is best thought of as common property since many of the ecosystem services it helps produce are non-rival and/or non-excludable. Private property regimes and markets alone are ineffective and inappropriate institutions to manage them sustainably. These systems can be better managed as commons, using more nuanced private and community property rights and Common Asset Trusts (CATs), with legal precedent in the Public Trust Doctrine. Effective CATs embody a generalized version of Elinore Ostromâ€™s eight core design principles for sustainable commons management: (1) shared identity and purpose; (2) equitable distribution of contributions and benefits; (3) fair and inclusive decision-making; (4) monitoring agreed behaviours; (5) graduated responses; (6) fast and fair conflict resolution; (7) authority to self-govern; and (8) collaborative relations with other groups and spatial scales. Here, we describe a few existing and proposed systems that approximate effective CATs. We also suggest how Costa Rica can transform its existing payment for ecosystem services (PES) scheme into a national CAT. Finally, we describe how CATs can facilitate more fair and effective public/private partnerships (PPPs) to invest in natural capital and ecosystem services.