Objective: To systematically review how process evaluations are currently designed, what methodologies are used and how are they developed alongside or within neurological rehabilitation trials.Methods: This mixed-methods systematic review had two evidence streams: stream I, studies reporting process evaluations alongside neurorehabilitation trials research and stream II, methodological guidance on process evaluation design and methodology. A search strategy was designed for each evidence stream. Data regarding process evaluation core concepts and design issues were extracted using a bespoke template.Evidence from both streams was analysed separately and then synthesised in a final overarching synthesis proposing a number of recommendations for futureresearch.Results: A total of 124 process evaluation studies, reporting on 106 interventions, were included in stream I evidence. 30 studies were included as stream II evidence. Synthesis 1 produced 9 themes, and synthesis 2 identified a total of 8 recommendations for process evaluation research. The overall synthesis resulted in 57 ‘synthesis recommendations’ about process evaluation methodology grouped into 9 research areas, including the use of theory, theinvestigation of context, intervention staff characteristics and the delivery of the trial intervention.Conclusions: There remains no consensus regarding process evaluation terminology within the neurological rehabilitation field. There is a need for process evaluations to address the nature and influence of context over time. Process evaluations should clearly describe what intervention staff bring to a trial, including skills and experience prior to joining the research. Process evaluations should monitor intervention staff’s learning effects and the possibleimpact that these may have on trial outcomes.