Mental ill health is very common. Most people with mental health problems live in the community, and as many as 1.5 million people in the UK may be involved in caring for a relative or friend with a mental illness or some form of dementia. Recent legislation and policy initiatives such as the National Strategy for Carers, and the National Service Frameworks for Mental Health and Older People emphasise the importance of providing support for this particular group of carers. The present paper reports the findings of a scoping study to identify what the research tells us about the effectiveness and cost-effectiveness of interventions for the carers of people with mental health problems, and also where there are gaps in the knowledge base. Some 204 evaluation studies were included in the review, just 13 of which had an economic component. The majority of studies were conducted in the USA, and were aimed at carers of people with Alzheimer disease or other forms of dementia. Overall, there was a lack of strong evidence to support any specific interventions, although almost all studies were able to identify some positive outcomes of services provided. In contrast to the relatively narrow approach to effectiveness adopted in most of the studies reviewed, the contributors to a consultation exercise perceived this concept in a far more rounded and holistic way. For them, the process of service delivery was as important as the outcome. There was relatively little research evaluating interventions and services singled out in UK policy initiatives as potentially useful in supporting this group of carers, and further evaluation studies are needed.