This review covers the key types of genetic research design, the methodology involved and emerging, and established findings in relation to child and adolescent psychiatry. Traditional family, twin, and adoption studies show that child and adolescent psychiatric disorders are familial and genetically influenced. Genes and environment contribute to all disorders. Genetic factors seem especially important for autism and attention deficit hyperactivity disorder. Twin and adoption study designs are now being used to examine gene-environment interplay, the effects of environmental risk factors, co-morbidity, phenotype definition, and developmental change. Molecular genetic strategies are increasingly being adopted to identify gene variants that increase risk of specific disorders. The ways in which specific gene variants exert risk effects at cellular and biological system levels are proving to be highly complex. There is also interest in examining the brain mechanisms that may be involved in risk pathways that link gene variant to psychopathology. Finally, molecular genetic studies also highlight the importance of gene-environment interplay, which seems to be especially important in depression and antisocial behaviour.