In the past, there was a common assumption that genetic and environmental influences were entirely separate and that, therefore, it was meaningful to ask whether particular mental disorders were mainly due to the one, or the other. Research has now shown that that is a misleading oversimplification because there are at least three different ways in which there is co-action between genes and environments. First, genes only have effects if the genes are ‘expressed’ in the tissues. Expression is influenced by multiple DNA elements, environmental forces and chance. Second, there are genetic effects on exposure to risk environments. Third, there are genetic effects on individuals’ vulnerability to environmental influences. In addition, it is clear that both genes and environments tend to have pleiotropic effects – meaning effects on multiple features. That means that it is misleading to view genes as ‘good’ or ‘bad’. These various considerations will be reviewed in terms of our understanding of mental disorders.