Biological innovations in agriculture did not enjoy protection by formal intellectual property rights (IPRs) for a long time, but the recent trend has been one of considerable broadening and strengthening of these rights. We document the nature and evolution of these IPRs and provide an assessment of their impacts on innovation. We integrate elements of the institutional history of plant IPRs with a discussion of the relevant economic theory and a review of applicable empirical evidence. Throughout this review, we highlight how the experience of biological innovation mirrors or differs from the broader literature on IPRs and innovation. We conclude with some considerations on the relationship between IPRs, market structure, and the pricing of proprietary inputs in agriculture.