Innovation is a major source of improvements in human welfare, and patent laws are intended to stimulate innovation. Yet the effects of patent laws on innovation are theoretically ambiguous. Strong patents that encourage pioneer inventors may discourage future innovators by reducing the pay-offs from their work. Economic history offers a wealth of empirical settings to study the effects of patent laws. Recent improvements in empirical methodology—through the creation of new data sets and advances in identification—have further improved the benefits of historical analyses. This chapter summarizes results from existing historical analyses of patent laws and highlights open questions.