Sleep problems are common in children, adolescents and adults and are often associated with psychiatric difficulties. Whilst a great deal of behavioural genetic research has focussed on understanding the genetic and environmental factors contributing exclusively to sleep disturbances and to psychopathology, an emerging body of literature centres on understanding the factors that contribute to the overlap between sleep disturbances and symptoms of psychopathology. The purpose of this chapter is to review some of the most important findings from behavioural genetic research which have assessed the aetiology of sleep-wake characteristics, sleep disorders and associations between sleep disturbances and psychopathology. We present findings from the following: (1) family studies of insomnia which provide an insight into possible familial factors involved in sleep disturbances; (2) quantitative genetic studies assessing the heritability of normal sleep patterns and specific sleep disorders; (3) quantitative genetic studies assessing the overlap in genetic and environmental influences between multiple sleep phenotypes and symptoms of psychopathology; (4) molecular genetic research, with a focus on associations between genes common to both sleep-related and psychopathology-related symptoms; (5) areas of research other than behavioural genetics which have provided an insight into possible shared environmental risk factors for sleep disturbances and psychopathology; and (6) studies positing sleep characteristics as plausible endophenotypes of psychopathology. Finally, we present a research agenda for behavioural geneticists within the sleep arena.