As people with Parkinson's experience progressive communication changes, effective, implementable speech and language therapy interventions are needed. Process evaluations alongside pragmatic randomised controlled trials are of clinical value if they describe, compare and understand the implementation of trial interventions. This paper reports the Parkinson disease COMM process evaluation. Parkinson disease COMM was a large, UK multi-centre phase III pragmatic RCT of SLT in the National Health Service. It recruited 388 people with Parkinson's who were randomised to Lee Silverman Voice Treatment, Standard NHS SLT, or no dysarthria intervention.