This chapter lays the theoretical foundations of long-run economic growth. After providing an overview of the three fundamental regimes that have characterized the process of development over the course of human history on the basis of the seminal work of Galor and Weil (Am Econ Rev 90:806–828, 2000), we review existing theories offering explanations of the different stages of development. In particular, we examine the predictions and underlying mechanisms of the traditional theories of economic growth and the theories of demographic transitions. We then show the relevance of the Unified Growth Theory to explain and capture the underlying mechanisms of the development process. Finally, we highlight the importance of integrating a gendered perspective in the study of long-run economic growth.