Scoping studies have been used across a range of disciplines for a wide variety of purposes. However, their value is increasingly limited by a lack of definition and clarity of purpose. The UK's Service Delivery and Organisation Research Programme (SDO) has extensive experience of commissioning and using such studies; twenty four have now been completed. This review article has four objectives; to describe the nature of the scoping studies that have been commissioned by the SDO Programme; to consider the impact of and uses made of such studies; to provide definitions for the different elements that may constitute a scoping study; and to describe the lessons learnt by the SDO Programme in commissioning scoping studies. Scoping studies are imprecisely defined but usually consist of one or more discrete components; most commonly they are non-systematic reviews of the literature, but other important elements are literature mapping, conceptual mapping and policy mapping. Some scoping studies also involve consultations with stakeholders including the end users of research. Scoping studies have been used for a wide variety of purposes, although a common feature is to identify questions and topics for future research. The reports of scoping studies often have an impact that extends beyond informing research commissioners about future research areas; some have been published in peer reviewed journals, and others have been published in research summaries aimed at a broader audience of health service managers and policymakers. Key lessons from the SDO experience are the need to relate scoping studies to a particular health service context; the need for scoping teams to be multi-disciplinary and to be given enough time to integrate diverse findings; and the need for the research commissioners to be explicit not only about the aims of scoping studies but also about their intended uses. This necessitates regular contact between researchers and commissioners. Scoping studies are an essential element in the portfolio of approaches to research, particularly as a mechanism for helping research commissioners and policy makers to ask the right questions. Their utility will be further enhanced by greater recognition of the individual components, definitions for which are provided.