Two approaches to studying opportunities have emerged in the entrepreneurship literature. This paper shows that the first of these approaches—which focuses on how alert entrepreneurs discover objective opportunities formed by exogenous shocks in an existing market—adopts a critical realist perspective. The second approach—which focuses on opportunities that are endogenously enacted by the actions of entrepreneurs themselves and do not have an existence independent of those human actions—adopts an evolutionary realist perspective. Differences between these epistemological assumptions have an important impact on opportunity research in the field of entrepreneurship, and thus are likely to have an important impact on the evolution of research, practice, and teaching in the field of entrepreneurship as a whole.