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Background: The Chronic Headache Education and Self-Management Study (CHESS) multicentre randomised trial evaluated the impact a group education and self-management support intervention with a best usual care plus relaxation control for people living with chronic headache disorders (tension type headaches or chronic migraine, with or without medication overuse headache). Here we report the process evaluation exploring potential explanations for the lack of positive effects from the CHESS intervention. Methods: The CHESS trial included 736 (380 intervention: 356 control) people across the Midlands and London UK. We used a mixed methods approach. Our extensive process evaluation looked at context, reach, recruitment, dose delivered, dose received, fidelity and experiences of participating in the trial, and included participants and trial staff. We also looked for evidence in our qualitative data to investigate whether the original causal assumptions underpinning the intervention were realised. Results: The CHESS trial reached out to a large diverse population and recruited a representative sample. Few people with chronic tension type headaches without migraine were identified and recruited. The expected 'dose'of the intervention was delivered to participants and intervention fidelity was high. Attendance ("dose received") fell below expectation, although 261/380 (69%) received at least at least the pre-identified minimum dose. Intervention participants generally enjoyed being in the groups but there was little evidence to support the causal assumptions underpinning the intervention were realised. Conclusions: From a process evaluation perspective despite our extensive data collection and analysis, we do not have a clear understanding of why the trial outcome was negative as the intervention was delivered as planned. However, the lack of evidence that the intervention causal assumptions brought about the planned behaviour change may provide some insight. Our data suggests only modest changes in managing headache behaviours and some disparity in how participants engaged with components of the intervention within the timeframe of the study. Moving forwards, we need a better understanding of how those who live with chronic headache can be helped to manage this disabling condition more effectively over time. Trial registration: ISRCTN79708100. [ABSTRACT FROM AUTHOR]