Background: Recent evidence demonstrates that children are not engaging in the recommended 60 min of moderate to vigorous physical activity per day. Physical activity (PA) interventions have been acknowledged by the WHO (2010) as a key strategy to increase the PA levels of children. School has been recognised as a primary location for reaching the majority of children and providing PA opportunities for them. However, the sedentary nature of lessons carried out in the classroom has been identified as a contributing factor to physical inactivity among this age group.Purpose: The aim of this study is to develop and evaluate a classroom-based intervention which integrates PA and academic content, and evaluate its effects on the PA levels of children aged 8-11 in Ireland.Methods: Active Classrooms is an 8-week classroom based intervention guided by the behaviour change wheel (BCW) framework (Michie et al. 2011) that will be evaluated using a cluster randomised controlled trial (RCT). Study measures will be taken at baseline, during the final week of the intervention and at follow-up after 4 months. The primary outcome is minutes of moderate-to-vigorous intensity physical activity during school time objectively assessed using accelerometers (Actigraph). Teachers' perceptions on the effectiveness and use of the intervention and students' enjoyment of the programme will be evaluated post intervention.Conclusions: Changing teacher behaviour towards using physically active teaching methods may increase the moderate to vigorous physical activity levels of their students. Therefore, the results of this study may have important implications for the health of children both now and into the future. (C) 2015 Elsevier Inc. All rights reserved.