Single sex schooling might appear to be an obscure issue on the sidelines of the educational policy debates of our times. But it is far from this. In fact, a sizable number of people and political organizations would like to make these schools obscure, but somehow they are “scaling up” rather than down. In 1996, there were only two public single sex schools operating in America. By 2015 there are now at least 100 public single sex schools, despite opposition from the outset. These schools are primarily serving poor, urban, black and Latino, at risk children. This book takes up the challenge of studying the effectiveness of single sex schools. Riordan frees the discussion of its ideological and political baggage and brings a degree of theoretical and empirical balance to the debate. The book provides a sociological foundation for considering single sex schools. The basic argument is that the larger school context of all girls or all boys serves as the driving factor for producing favorable outcomes in single sex schools.