There is a pressing need for rehabilitation researchers to further develop their understanding around the complex interventions they deal with on a daily basis. Prevailing methodological approaches, such as pragmatic RCTs, often fail to address the challenges resulting from the complexity of these neurological rehabilitation interventions, specifically in relation to fidelity. Furthermore, the need for an evidence-based client centred approach has inevitably led to an impact on the fidelity of trialled rehabilitation interventions and therefore a potential impact on trial outcomes. Process evaluations are regarded as effective at investigating why and how does an intervention work or fail. To date, there is very limited guidance to help rehabilitation researchers design and conduct one. Thus, there is a strong need to advance the development of process evaluation research within the field of neurological rehabilitation.