Most studies in the social and life sciences aspire to make inferences beyond their specific study sample(s), but they neither rigorously nor credibly deliver. It need not be so. In this book, we articulate the core conceptual dimensions of external validity and provide a framework for understanding each study’s external validity through three evaluative criteria: model utility, scope plausibility, and specification credibility. To ensure practicality and uptake of our evaluative criteria, we provide method-specific applications for experiments, natural experiments, quantitative observational methods, qualitative observational methods, and research synthesis approaches. We conclude with reporting standards that aim to make external validity an equal partner with internal validity/causality in the quest for scientific knowledge.