Background: Co-research with people with intellectual disability is a distinct form of patient and public involvement (PPI). This systematic review summarize published studies and protocols to report on the process of co-research in social and healthcare research. Method: Relevant studies were identified using electronic searches on ASSIA, PsycInfo and MedLine. Study quality was assessed, and information relevant to the process of working with co-researchers was extracted and thematically analysed. Results: Thirteen studies were retrieved. Data are reported under three themes: (i) challenges of co-research; (ii) facilitators of co-research; (iii) benefits of co-research. Best practice is presented as a model of co-research. Content analysis on 12 research protocols identified four themes related to PPI. Conclusions: All stakeholders involved in co-research with people with intellectual disability can benefit, providing there is adequate infrastructure to accommodate and empower the co-researchers. Many current intellectual disability research projects still lack systematic involvement of PPI members.