This article is the second in a series to celebrate the 70th anniversary of the ILR Review. The series features articles that analyze the state of research and future directions for important themes this journal has featured over many years of publication. In this article, the authors assess the credibility of research that has tested the theoretical contests between the monopoly and the collective voice model of unions developed by Freeman and Medoff in What Do Unions Do? The authors go beyond prior analyses by examining more than 2,000 estimates that consider the effects of unions on a broad range of organizational and individual outcomes, including productivity, productivity growth, capital investment, profits, and job satisfaction. They advance our understanding of the current empirical findings and credibility of this research by using meta-statistical analysis to evaluate research quality, publication selection bias, statistical power, and heterogeneity. The authors conclude that compared to other areas of economics, research on union effects has lower bias but larger problems of statistical power. They argue that Freeman and Medoff’s monopoly–collective voice model helped produce more credible results, and they suggest ways to reduce the power and heterogeneity problems in existing research.