Quantitative genetics, using data from twin and adoptee studies, has shown substantial genetic influences on all forms of psychiatric disorder; however, with just a few exceptions, the evidence indicates that the disorders are multifactorial, with influences that are both genetic and environmental. In recent years, molecular genetics has begun to identify individual susceptibility genes; examples are given for schizophrenia, attention deficit/hyperactivity disorder, and Alzheimer's disease. Both quantitative and molecular genetics have shown the importance of gene-environment interplay with respect to the commoner disorders of emotions and behaviour. In particular, it has been found that genetic influences moderate people's vulnerability to environmental risks. Five main alternative routes by which genes indirectly (via their effects on proteins) lead to multifactorial psychiatric disorders are described. Four main research issues are highlighted: the fuller delineation of the mechanisms involved in nature–nurture interplay and its role in aetiology; determination of how genes play a role in the neural underpinning of psychiatric disorders; identification of the ways in which genes suggest a dissection of disorders; and an understanding of the role of risk dimensions and disorder dimensions.