This article reports on a systematic review of the patient and public involvement (PPI) literature in the UK that was undertaken in 2005 to support an evaluation of the Patient Advice and Liaison Services (PALS) in England.The general PPI literature has been extensively reviewed; here we focus on the relatively small empirically based evaluation literature.The research adopts the framework of realistic evaluation, which identifies contexts, mechanisms and outcomes that lead to favourable and unfavourable outcomes. The question guiding the review is therefore: 'What context and mechanism factors can be identified from empirical research that will lead to successful PPI outcomes?'. © The Policy Press.