“Qualitative Comparative Analysis” (QCA) is an increasingly applied methodological tool in comparative social sciences. It is well suited for the analysis of causally complex claims framed in terms of necessity and sufficiency. This article presents the epistemology of QCA and discusses its applicability to social science research questions. It also illustrates some of the features that have recently been added to this set of methodological tools. This article is best read in close conjunction with Schneider and Wagemann’s “Standards of Good QCA Practice,” the next paper in this journal issue.