This project offers an innovative history of health inequality during the 20th century by focusing on infant mortality in three industrial cities: Chicago in the United States, Lille in France, and Manchester in Britain, from 1900 to 1980. If economists and political scientists have explained the rise and fall of inequality, they have mainly used macro-quantitative data that tends to overemphasize structural trends. This generalization overlooks social and local differences and puts too much emphasis on the period from the 1930s to the 1970s as an age of increasing equality. By combining the tools of social history and social epidemiology across national contexts, this project will add social and local “thickness” to the history of inequality and provide a new perspective on the impact of social policy through the lens of infant mortality. We believe that lessons that can be learned about the successes and limits of the struggle against infant mortality in major industrial cities can prove helpful today.