Briefing for potential Case Studies. Background and introduction to the study: This study has been funded by the NIHR Health Services Research programme to help develop the evidence base on the impact of public involvement in research. The commissioning of this research follows two recent literature reviews (Staley 2009; Brett et al. 2010) which demonstrates gaps in the evidence base for the impact of public involvement in research. The aim of this project is to assess the impact of public involvement in research through a 'realist evaluation' (Pawson & Tilley 2008) which emphasises the importance of hypothesising regularities of context, mechanism and outcome (CMO). We will examine the implementation of public involvement by asking what context factors (e.g. institutional support) enable which mechanisms (e.g. training) to lead to what outcomes (e.g. improved research design). We will then compare CMO regularities in eight case studies with a diverse range of research projects (e.g. different research subjects, methods, stages of the research process, scale of the research, approaches to involvement). The Research will be undertaken by academic researchers from the University of the West of England and Coventry University in partnership with members of the public working as 'research partners'. Each case study will be undertaken jointly by an academic researcher and a research partner. As well as this NIHR project, one research partner co-applicant (Rosie Davies) is also conducting a linked ethnographic study as part of her PhD of three case studies which will involve more in-depth observation of research team activities