In disciplines such as sociology, the meaning and interpretations of key terms are debated with great passion. From foundational concepts (e.g., class and structure) to more recent ones (e.g., globalization and social capital), alternative definitions grow organically from exchanges between competing researchers who inherit and then strive to strengthen the conceptual apparatus of the discipline. For the methodology of social inquiry, similar levels of contestation are less common, presumably because there is less scope for dispute over matters that many regard as mere technique. The terms causality and causal are the clear exceptions. Here, the debates are heated and expansive, engaging the fundamentals of theory (What constitutes a causal explanation, and must an explanation be causal?), matters of research design (What warrants a causal inference, as opposed to a descriptive regularity?), and domains of substance (Is a causal effect present or not, and which causal effect is most important?). In contrast to many conceptual squabbles, these debates traverse all of the social sciences, extending into most fields in which empirical relations of any form are analyzed. The present volume joins these debates with a collection of chapters from leading scholars.