Decades of research across several disciplines have produced substantial evidence that labor unions, on balance, reduce economic disparities. But unions are complicated, multifaceted organizations straddling markets and politics. Much of their equality-promoting influence occurs through their ability to reduce class-based inequity in politics and public policy. Declining unionization across much of the developed world is eroding workers’ bargaining power. Reduced economic leverage puts pressure on union solidarity and weakens labor-based political movements. Important research design problems and significant heterogeneity across unions, regions, countries, and time imply a continued need for more work.