Service user involvement in the planning and provision of mental health services has been growing over the last two decades, especially in countries and areas where institutional service provision has been changed to a community-orientated model of care. However, the material involvement of service users in mental health research is still in its infancy. The aim of this paper is to attempt to place these developments in a conceptual context, to summarise the ethics-based and evidence-based reasons why it has to be considered as necessary, and to illustrate some of the emerging evidence which shows the advantages to be gained from it. In particular the results of recent studies are briefly reported, showing that outcomes data rated by service users in some cases are more important than those rated by staff. The reduction in patient-rated unmet needs in the social domain was the strongest predictor of an increase in subjective quality of life. The importance of including service user preferences within the content of the research questions is exemplified by the results of a recent study that showed that joint crisis plans can significantly reduce the use of compulsory admission during crises and by a review that demonstrated that the use of an explicit service user perspective produced distinctive insights into the long-term effects of Electro-Convulsive Therapy (ECT).