Within community-engaged research, education and social care, peer models that partner with local "insiders" are increasingly common. Peer models are composed of insider "lay" community members who often share similarities or background with a project's target population. Peers are not academically trained, but work alongside researchers and professionals to carry out specific tasks within a project, or in the truest sense of partnership, peers collaborate throughout the project from start to finish as an equal member of the team. Although peer models are used widely, the literature lacks consistency and clarity. This systematic review of literature used a qualitative thematic synthesis to examine and report how, where and why peer models have been used in research, education and social care. We examined the language and titles used to describe the peers, details of their involvement in community-engaged projects, the setting, content/topic of study, level of engagement and related benefits/outcomes of such models. Focusing on the last 10 years, we conducted a comprehensive literature search twice between September 2016 and June 2017. The search resulted in 814 articles which were assessed for eligibility. Overall, 251 articles met our inclusion criteria and were categorised into three categories: empirical (n = 115); process/descriptive (n = 93); and "about" peers (n = 43). Findings suggest that there is a wide variety of peers, titles and terminology associated with peer models. There is inconsistency in how these models are used and implemented in research studies and projects. The majority of articles used an employment peer model, while only a handful involved peers in all phases of the project. The results of this literature review contribute to understanding the use, development and evolution of peer models. We highlight potential benefits of peer models for peers, their communities and community-engaged work, and we offer recommendations for future implementation of peer models.