The impact of unions on productivity is explored using meta-analysis and meta-regression analysis. It is shown that most of the variation in published results is due to specification differences between studies. After controlling for differences between studies, a negative association between unions and productivity is established for the United Kingdom, whereas a positive association is established for the United States in general and for U.S. manufacturing.T HE RELATIONSHIP BETWEEN UNIONS AND PRODUCTIVITY has attracted considerable attention from scholars in industrial relations and economics, as well as from policymakers, unions, and business in general. Despite voluminous theoretical literature, controversy continues regarding the impact of unions on productivity, as well as on other aspects of business, such as employment, research and development (R&D), profitability, and investment. In traditional economic analysis, unions are said to distort labor market outcomes through, for example, legal and custom-driven restrictions on relative wages, the imposition of employment restrictions, and protection against layoffs. Unions are said also to be a contributing factor to aggregate as well as sectoral unemployment and the associated output losses. In contrast to these arguments, Freeman (1976) andFreeman andMedoff (1984) argued that unions can raise productivity by providing workers with a means of expressing discontent as an alternative to "exiting," by opening up communication channels between workers and management, and by inducing managers to alter methods of production and to adopt more efficient policies.The controversy in the theoretical literature is matched by controversy in the empirical literature. Empirical findings are divided between positive and negative union-productivity effects, and many studies cannot reject the hypothesis of a zero effect. Hence generalizations from the available evidence * The authors' affiliations are, respectively,