Collaborative care aims to improve the physical and mental health of people with severe mental illness (SMI). Common to all definitions is that collaborative care aims to develop closer working relationships between primary care (family doctors or GPs and practice nurses) and specialist health care (such as Community Mental Health Teams). There are different ways in which this can be achieved, making collaborative care very complex. Integrating or joining‐up primary care and mental health services, so that they work better together, is intended to increase communication and joint working between health professionals (e.g. GPs, psychiatrists, nurses, pharmacists, psychologists). This is meant to provide a person with severe mental illness with better care, based in the community, which is often a less stigmatised setting than hospital, and that promotion of good practice helps consumers maintain contact with services. A major issue is that many GPs still feel that physical health problems (such as diabetes, heart disease, smoking cessation) are more their concern and see treatment of severe mental illness as the job of psychiatrists and other mental health professionals. Collaborative care aims to improve overall quality of care by ensuring that healthcare professionals work together, trying to meet the physical and mental health needs of people. The aim of the review was to assess the effectiveness of collaborative care in comparison to standard or usual care. An electronic search for trials was carried out in April 2011. The primary focus of interest was admissions to hospital. According to the one included study in this review, collaborative care may be effective in reducing going into hospital (less psychiatric admissions and other admissions). It also helps improve people’s quality of life and mental health. However, all evidence was either low or very low quality and further research is needed to determine whether collaborative care is good for people with SMI in terms of clinical outcomes or helping people feel better as well as its cost effectiveness.