Physical activity is a modifiable risk factor for several physical and mental health conditions. It is well established that people with severe mental illness have increased risk of physical health complications, particularly cardiovascular disease. They are also more likely to be physically inactive, contributing to the elevated cardiovascular and metabolic risks, which are further compounded by antipsychotic medication use. Interventions involving physical activity are a relatively low risk and accessible way of reducing physical health problems and weight in people with severe mental illness. They also have wider benefits for mental health symptoms and quality of life. However, many barriers still exist to the widespread implementation of physical activity interventions in the treatment of severe mental illness. A more concerted effort is needed to facilitate their translation into routine practice and to increase adherence to activity interventions