Public opinion about labor unions has long been viewed as an important determinant of industrial relations outcomes. Yet, analyses of changes in union popularity over time have been largely qualitative and have focused on the impact of short-term idiosyncratic events. This paper provides a quantitative analysis of the determinants of American public approval of unions from 1936 to 1991. Hypotheses relating to the union wage advantage, strike activity, the national unemployment rate, and World War II, receive the strongest support. The implications of these results for organized labor and future research on attitudes toward unions are discussed.