This article surveys recent data and empirical literature on (trends in) unionization and its determinants in advanced countries. There are some robust stylized facts, for instance that unionization is positively related to public sector employment, establishment size, and the business cycle. Union-administered unemployment insurance and union presence at the workplace also play a positive role. However, some seemingly obvious explanations for union decline – such as economic globalization and changes in economic structure and workforce composition – do not seem to have impeded union membership everywhere; nor has the decentralization of collective bargaining. It also remains an open question whether changes in social values and employee attitudes towards unions have affected unionization negatively.