The importance of involving patients and the public in health care research is globally recognized, but how best to do this in critical care is unclear. The aim of this first published review was to explore the extent and nature of evidence on service user involvement in critical care research and quality improvement. Using the scoping review framework described by Arksey and O'Malley (2005), a team of service user and critical care researchers searched eleven online databases, reviewed relevant web sites, conducted forward and backward citation searching and contacted subject experts. Extracted data were subjected to a narrative synthesis based on the objectives of the review. Findings from a broad range of evidence support that involvement is becoming more commonplace and that experiences are generally positive. Data extracted from 34 publications identify that involvement is most commonly reported at the level of consultation or participation in project teams, however, the extent to which involvement impacts on projects output remains unclear. Key barriers and facilitators relate to the challenge of recruiting a diverse group of service users, dealing with power hierarchies, being adaptable and effective consideration of the resource requirements. More research is required to identify the most effective methods to support the opportunity for involvement and more thorough reporting of service user involvement practices is strongly recommended.