What is the optimal system of intellectual property rights to encourage innovation? Empirical evidence from economic history can help to inform important policy questions that have been difficult to answer with modern data: For example, does the existence of strong patent laws encourage innovation? What proportion of innovations is patented? Is this share constant across industries and over time? How does patenting affect the diffusion of knowledge? How effective are prominent mechanisms, such as patent pools and compulsory licensing, that have been proposed to address problems with the patent system? This essay summarizes results of existing research and highlights promising areas for future research. PUBLICATION ABSTRACT