Muslim Communities Learning About Second-hand Smoke (MCLASS-II) in Bangladesh is an effectiveness-implementation hybrid study funded by the Medical Research Council, UK.  Its overall aim is to reduce the burden of disease due to second-hand smoke in low- and middle-income countries by discovering innovative community-based approaches to behaviour change. It will assess the feasibility and acceptability of introducing an intervention using imams as a change agent, thereby helping policy makers adapt tobacco cessation strategies to the local context. The findings will help researchers conduct a randomised controlled trial to assess the effectiveness of the intervention in reducing exposure to second-hand smoke at home.