PROJECT SUMMARY Non-alcoholic fatty liver disease (NAFLD), characterized by increased intrahepatic triglyceride (IHTG ≥5% of liver volume) accumulation, is strongly associated with obesity, insulin resistance (IR), and decreased cardiorespiratory fitness. Untreated NAFLD can lead to progressive hepatic damage. Predisposing factors leading to poor outcomes are not well-characterized. Although IR is thought to be central to the development of NAFLD, not all subjects with IR develop NAFLD. However, the degree of IR positively associates with IHTG in those with NAFLD. Cardiopulmonary fitness level, independent of IR, negatively associates with IHTG. Additionally, children and adults with NAFLD have unfavorable circulating lipid profiles and elevated markers of inflammation, which put them at risk for future cardiometabolic complications. Using magnetic resonance imaging, we showed that 40% of children with obesity who have entered a clinical weight management program had NAFLD with varying degrees of IHTG. Children with NAFLD, in this cohort, had higher HOMA-IR values (a surrogate marker for IR) and lower fitness levels as assessed by continuous treadmill graded protocol. Despite the high prevalence and adverse outcomes of NAFLD, there is no approved pharmacotherapy for treatment. Clinical guidelines recommend weight loss via lifestyle modifications, although this is not easy for most to achieve and sustain. Recent studies demonstrate that exercise alone can effectively decrease IHTG and IR in adolescents, even in the absence of significant weight reduction. The beneficial effects of exercise in NAFLD could also be independent of IR. Studies in children with NAFLD are scarce, and how exercise improves NAFLD remain elusive. Based on available literature and our own pilot data, we hypothesize that an exercise-driven decrease in IHTG in children with NAFLD will be positively associated with increase in fitness and decrease in IR levels. Here we propose a 4-week high-intensity interval training (HIIT) intervention vs. no exercise in adolescents with NAFLD. As a practical and time-saving regimen, HIIT is a well-tolerated exercise strategy with significant metabolic benefits. Our long term goal is to determine whether exercise is beneficial for treating pediatric NAFLD and, if so, to identify (i) the characteristics of the participants with a positive response and (ii) the mechanistic pathways implicated in the development and progression of NAFLD (i.e., insulin resistance, etc.) and, by which exercise improves NAFLD. The findings of this study will inform clinical providers in the use of HIIT as a modality to treat adolescent NAFLD and associated morbidities, and produce preliminary data for mechanistic studies to probe molecular pathways implicated in the pathogenesis of NAFLD.