People with severe mental disorders live on average 10 to 20 years less than the general population. This huge gap is largely explained by an increasing prevalence of cardiovascular disease, diabetes mellitus and metabolic syndrome. Risk factors include limited physical activity and sedentary behaviour. Increasing physical activity and sedentary behaviour play a crucial role in preventing these comorbidities, and thus addressing the mortality gap. Research shows the potential benefit of interventions targeting physical activity and sedentary behaviour for people with severe mental disorders. However, translation into clinical practice has been slow. One possible explanation is the many barriers they experience. Evidence on the effectiveness of such interventions is insufficient for policy-making. There is also a need for information on cost-effectiveness. This evidence can be obtained from health economic evaluations.