There is consistent evidence that major depression is familial and population-based twin studies as well as hospital register-based twin studies find substantial heritability. However, there is also a large proportion of variation in liability left to be explained by nongenetic factors. Although there seems little doubt that life events play a role in precipitating depression, studies that have attempted to examine familial liability along with social adversity find that environmental measures tend to be contaminated by genetic effects. Thus, the tendency to experience (or report) life events appears to be influenced by shared family environment, and for certain types of events there is a genetic component. The molecular genetic basis of liability to depression is an under-researched area, but some candidate gene studies show potentially promising results. Systematic mapping studies aiming to cover the entire genome are currently being launched.