Theory building from multiple cases has generated some of the most cited and intriguing research over the last 80 years. Yet there remains confusion regarding how to judge its rigor. We begin with a short history and description of the method including replication logic. We then focus on four central criteria for rigor: The first is strong emergent theory – i.e., theory that is accurate, internally coherent and parsimonious. Such theory usually includes clear constructs and logical underlying arguments for relating them as well as sensitivity to boundary conditions and alternative explanations. The second is compelling evidence that starts with deep immersion in rich data from multiple sources and informants, and uses “construct tables” to analyze and display data. The third is theoretical sampling and research designs that involve careful selection of cases using research designs that provide a sharp focus on the phenomena of interest. The fourth is an intriguing research question that often emphasizes processes, centers on either similarity or variance, and emerges from a precise reading of literature. We briefly note that rigor is not based on superficial criteria like narrow writing formats, rigid rules for data analysis, and exhaustive details of the method. We conclude that rigor in theory building from multiple cases rests on the same few core criteria that characterize, in a broad sense, most if not all rigorous research.