This article both describes and explains gendered patterns of industrialization across 27 sectors and 10 countries in three regions. Contrary to common perceptions that women's participation in manufacturing work is to be explained primarily by economic or cultural variables, I demonstrate the central role of an additional variable - the strength of unions - in delaying the entry of women into the manufacturing workforce. I argue that cross-national differences in gendered patterns of industrialization are intimately tied to the balance of employment in labor-intensive versus capital-intensive sectors, employment growth, fertility, and the strength of labor unions. Surprisingly, this study finds that supply variables have weak effects on feminization. Demand-side factors and the power of unions have stronger and more consistent effects on feminization than cultural factors that shape the supply characteristics of female labor.