Summary Physical activity has been shown to enhance children's executive function (EF) that is fundamental for reasoning, problem solving, and academic achievement. High-intensity interval training (HIIT), a form of exercise consisting of short intense exercise bouts separated by recovery intervals, has gained considerable attention due to its time-efficiency for increasing physical activity and fitness in an enjoyable and reinforcing manner while generating chronic and acute EF benefits in children. However, none of the existing programs utilized the recovery intervals during HIIT, which typically consist of passive and mindless breaks, despite evidence indicating larger cognitive benefits from exercise combined with cognitively engaging activity. The recovery intervals during HIIT provide an opportunity for mindfulness (MF), a mental exercise of self- awareness and self-regulation that improves EF. Repeated practice on the flexible regulation of attentional focus and bodily movement between mindful recovery and exercise bouts is theoretically sound for maximizing long-term cognitive gains because the added MF activity provides enriched experiences and cognitive challenges that stimulate the development of EF. HIIT and MF also have similar acute benefits to EF yet differential impacts on neuroelectric correlates of conflict processing and updating of mental representation. Thus, HIIT and MF may be complementary as they alter differential functional neural underpinnings of EF. This proposal aims to test the effectiveness of a novel mindful HIIT intervention for improving EF in 10 - 12 years old children. By using two active control interventions delivering HIIT without MF and delivering MF without HIIT as well as a passive control intervention delivering sedentary activities without inducing MF, we will further determine the unique effects of HIIT, MF, and their combination on children's EF. Specifically, this proposal will conduct a 12-week school-based cluster-randomized controlled trial and a laboratory-based acute intervention cross-over trial to investigate the long-term and short-term effects of mindful HIIT intervention on children's EF. Our central hypothesis is that integrating MF into HIIT will generate larger long-term and short-term positive effects on children's EF than MF or HIIT intervention along. Outcomes consistent with our hypotheses will provide novel evidence indicating the effectiveness of chronic and acute engagement in mindful HIIT for enhancing children's EF. The successful completion of this study will serve to guide future randomized controlled trials testing the mechanisms and dose-response associations of the long- term and short-term mindful HIIT interventions with behavioral and neural outcomes of childhood EF. This research agenda will support the development of scalable and sustainable interventions that uses MF technique to maximize the positive impacts of exercise on childhood cognitive and brain development wh...