Background Advance care planning (ACP) is reported to improve the quality of outcomes of care among those with life-limiting conditions. However, uptake is low among people living with multiple sclerosis (MS) and little is known about why or how people with MS engage in this process of decision-making. Aims To develop and refine an initial theory on engagement in ACP for people with MS and to identify ways to improve its uptake for those who desire it. Methods Realist review following published protocol and reporting following Realist and Meta-narrative Evidence Synthesis: Evolving Standards (RAMESES) guidelines. A multidisciplinary team searched MEDLINE, PsychInfo, CINAHL, Scopus, Web of Science, Embase, Google Scholar in addition to other sources from inception to August 2019. Quantitative or qualitative studies, case reports, and opinion or discussion articles related to ACP and/or end of life discussions in the context of MS were included, as well as one article on physical disability and one on motor neuron disease, that contributed important contextual information. Researchers independently screened abstracts and extracted data from full-text articles. Using abductive and retroductive analysis, each article was examined for evidence to support or refute 'context, mechanism, and outcome' (CMO) hypotheses, using the Integrated Behaviour Model to guide theory development. Quality was assessed according to