The document presents a framework for studying the optimal design of contractual property rights using mechanism design. It discusses how property rights determine agents' outside options in economic interactions and impact ex-post efficiency and investment incentives when the social planner cannot commit to future mechanisms. The authors analyze how to design property rights to alleviate these frictions in a setting with one-sided private information and bargaining power. A key result is that the optimal property right is often simple but flexible, featuring an option to own the resource.