The idea of mechanism-based explanation has a long history in the social sciences. This article examines two influential early sources of this idea that have become almost invisible in current debates about social mechanisms. The first is Rom Harré’s realist philosophy of natural science, which articulated an account of mechanisms that provided the foundation for later critical realist ideas about causal explanation. Harré’s metaphysical theory about causation consists of a tight conceptual package that involves ideas about powers, natures, essences, and generative processes. After describing its core ideas, the article shows how Roy Bhaskar adapted them to the social sciences and discusses why Harré himself did not make the same move. The second source of mechanical inspiration in the social sciences is Jon Elster, whose approach is quite different from that of Harré and Bhaskar. The article describes how Elster’s definition of mechanism has changed over time to reflect his theoretical concerns and how he introduces the important idea of a toolbox of mechanisms.