Female kindergartners come to school with reading skills that are slightly more advanced, on average, than those of males. They are also less likely to have developmental difficulties and are more likely to exhibit good social skills and classroom behavior. The higher frequency of behavior and adjustment problems that males exhibit when entering kindergarten foreshadows the greater number of males who experience conduct and disciplinary problems later in elementary and secondary school (Coiro, Zill, and Bloom 1994). Though some of the early problems may be transitory and simply reflect different developmental trajectories for boys and girls, others may be predictive of later and more serious disturbances. In contrast, despite the equivalent mathematics skills and general knowledge and better behavioral adjustment that females typically display at school entrance, females lag behind males in mathematics and science achievement in the later grades (NCES 1998). It is possible that this pattern is related to differences in development and social roles between the sexes that emerge as children reach adolescence. The longitudinal data on curriculum, instructional practices, and achievement that the ECLS-K will produce as it follows boys and girls through elementary school should be of value in exploring these questions.