INTRODUCTION Waterpipe tobacco smoking is increasing globally particularly among youth. In Lebanon, the high prevalence of waterpipe tobacco smoking among younger age groups calls for immediate intervention particularly given its negative health effects. To date, such interventions have rarely been implemented or evaluated. METHODS This manuscript describes the process evaluation of a school-based intervention to prevent/delay waterpipe tobacco smoking among 6th and 7th graders (n=844) in Lebanon. Process evaluation documents whether an intervention is implemented as planned, and guides understanding of the relationship between the intervention activities and outcomes. The intervention was carried out over 5 months during 2011-2012 on school premises and during school hours, using a participatory approach. The ten intervention sessions included knowledge, skills and a social promise. The process evaluation assessed fidelity, dose delivered, dose received, reach, satisfaction, and the influence of context. Tools included observation and satisfaction forms, attendance log sheets, focus group discussions, and daily journal entries. RESULTS The majority of participants (87.2%) attended at least 75% of the sessions. Results indicate high fidelity of implementation; 72.3% of the activities were rated by facilitators to be fully implemented -with high participant satisfaction; 89.5% of children chose a happy face to express their rating of the session. Facilitators rated children's participation as 'positive and active' in 77.9% of the sessions. CONCLUSIONS Main challenges to implementation were contextual at the country and school level, and related to local pro social norms around waterpipe tobacco smoking. The experience of this intervention confirms the critical importance of context in program implementation.