Quantitative genetic research includes a range of genetically sensitive research designs that rely on family samples to study the relative importance of genes and environments for individual difference in psychopathology. The past decades have seen an increase in quantitative genetic research focused on the origins of childhood and adolescent psychopathology. Evidence from this research univocally demonstrates that genetic factors play an important role in all forms of psychopathology, and that these genetic factors interact with the environment to shape the development of childhood and adolescent psychopathology. The goal of this chapter is to highlight how recent methodological developments and the accumulation of longitudinal data now allow quantitative genetic research to go beyond asking “if” genetic factors are important, to instead address important questions regarding gene-environment interplay in the development of childhood and adolescent psychopathology. It begins by introducing the family, adoption, and twin designs, and summarizing the main findings from these methods for child and adolescent psychopathology. It then provides concrete examples of how multivariate and longitudinal quantitative genetic research designs can be used to address important questions regarding etiology across different levels of symptom severity, comorbidity, and development, and to study gene-environment interplay in child and adolescent psychopathology. It concludes by highlighting important outstanding questions in childhood psychopathology that need to be addressed in future quantitative genetic research.