Background: Children's and young people's emotional wellbeing is in decline and models of good practice are needed to help schools address the needs of vulnerable pupils. Effectiveness studies have provided empirical evidence for school-based socio-emotional interventions but few examine process issues to explain procedures and mechanisms underlying behavioural change. Aims: This paper explores behaviour change drivers elicited from a qualitative study of the Pyramid Club socio-emotional intervention; to gain an understanding of underlying processes and, thereby, determine the factors that contribute to behaviour change in programme recipients. Behaviour change drivers represents one of five global themes yielded from the complete dataset. Participants: Participants comprised a total of 65 young people aged 11-14 years (24 males; 41 females) who had attended one of eight Pyramid clubs at their respective school and 23 club leaders who had helped to run a club. Methods: Focus groups (in each school) were used to collect data from participants; groups for Pyramid attendees and club leaders were run separately. Data were thematically analysed jointly. Findings: Behaviour change drivers were organised into two thematic categories--Behaviour change procedures, which comprise contextual elements favourable to programme effectiveness and Behaviour change techniques, specific mechanisms prompting behaviour change. Conclusion: Findings extend Pyramid Club's existing evidence base and can be used to better inform decision-makers in schools making implementation choices. More qualitative studies are needed to augment findings from effectiveness studies and to enable future programme development.