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 future research.

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, the investigation 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 possible impact that these may have on trial outcomes.