Although involvement of the consumer is increasingly being advocated in health related research, it is not welcome universally. Furthermore, the underlying rationale is rarely made explicit. Policy makers, health care professionals, and researchers need to be clear about the benefits and ways of including lay perspectives and the criteria for evaluating these. Examples of lay involvement in setting research agendas, [1][2][3][4] methodological debate, 5 and specific projects 4 6 7 are accumulating, but little clear evidence about the benefits and costs of different ways of incorporating lay input into health services research is available.We outline two basic reasons for incorporating lay perspectives into research and discuss some common objections. A framework is offered to help clarify the dimensions of lay involvement in health research. We use the term "lay" to mean people who are neither health care professionals nor health services researchers, but who may have specialised knowledge related to health. This includes patients, the general public, and consumer advocates.