The Arizona Twin Project is an ongoing longitudinal study designed to elucidate gene-environment interplay underlying the development of risk and resilience to common mental and physical health problems during infancy, childhood, and adolescence. The Arizona Twin Project spans individual, family, neighborhood, and sociocultural levels of analysis, with an emphasis on sleep, pain, physiological stress processes, mental and physical health, and academic competence. Importantly, our ethnically diverse sample is representative of the state of Arizona, thus affording the opportunity to consider the role of culture in the etiology of various child outcomes.