This volume reassesses the problem of explanation in social science from what remains a marginalized, realist perspective. Because the problem of explanation is central to inquiry in social science, the volume also provides a systematic philosophy of social science. It begins with the idea that the fundamental goal of theory in both the natural and social sciences is not, contrary to widespread opinion, prediction and control, or the explanation of events (including “behavior”). Rather, more modestly, theory (at least in one of its clear senses) aims to provide an understanding of the processes which jointly produce the contingent outcomes of experience. We understand why the planets move in ellipses, why materials burn, and why salt dissolves in water (if and when it does) when we have a physical theory that provides a causal mechanism. By providing the principles detailing the nature of molecules, the atomic structure of salt and water, the principles of their action, and so on, we can understand combustion and solubility – and other chemical processes. Indeed, while the theoretical work of physical scientists often begins with the effort to understand patterns, they are not interested in, nor generally capable of, providing either “explanations” or “predictions” of particular events. For example, the trajectory of a boulder splintering as it rolls down a hill is fully understood in terms of physical principles, but neither the trajectory nor the final positions of the splintered parts can be explained or predicted.