There are marked sex differences in the prevalence of most types of child psychopathology. Understanding the causes of these sex differences has the potential to advance causal theories of child psychopathology. The goal of this special section is to stimulate rigorous research focused on the causes of sex differences in the prevalence of child psychopathology. To provide a context for the articles included in this special section, a brief summary of recent findings of sex differences in developmental trajectories of child psychopathology is presented along with a discussion of methodological issues and promising directions for future research on sex differences.