The interrelationships between physical illnesses and mental disorders are diverse and complex. People with severe mental illness, particularly those with psychotic and affective disorders as well as pronounced personality disorders (serious/severe mental illness, SMI), have an increased risk of mortality compared to the general population as well as a higher prevalence of numerous somatic comorbidities. In addition to disease-related factors, explanation approaches for this also relate to the healthcare system. In routine psychiatric and psychotherapeutic care it is very important to keep an eye on the somatic health of people with SMI as physical impairments can easily be overshadowed by the mental illness. This article summarizes selected aspects of the broadly covered topics taking epidemiological data into account and with a focus on collaborative medical care of frequent diseases.