The co-occurrence of physical and mental disorders occurs more frequently than by chance. When there is multi-morbidity, this places a significant burden on the individual with the conditions with worsened clinical outcomes for both physical and mental disorders. The underlying reasons for the association are multifactorial but include factors that create a ‘common soil’ such as genetics, early intra-uterine development and social determinants of health. Adult health behaviours also play an important role and provide an opportunity for prevention. Health inequalities faced by people with mental illness, such as access and availability of high-quality health care, need to be reduced if holistic care is to be improved.