PROJECT SUMMARY/ABSTRACT: Family-based lifestyle interventions that combine dietary modification, physical activity (PA), and behavioral support are the standard of care for pediatric obesity treatment; however, methodologic heterogeneity across studies make it difficult to determine which specific strategies are most efficacious. Although interventions targeting PA alone result in limited weight loss, the health benefits of PA are extensive; and, interventions that combine PA training with dietary changes appear to have the most robust impact on weight. Thus, PA remains a key therapeutic target for the primary treatment of pediatric obesity. The importance of effective interventions that target PA may be particularly salient during adolescence, when PA levels decline dramatically. Although guidelines endorse the importance of regular, continuous moderate-to- vigorous PA, the ideal components of an age-appropriate activity program, including the required exercise intensity, have not yet been well-established, particularly for adolescents. High-intensity interval training (HIIT) which consists of repeated brief bouts of high-intensity PA with intervening short periods of recovery may be particularly efficacious in improving health as it requires less time than more traditional aerobic training. Despite a growing literature that supports HIIT training as safe and effective in improving body composition and cardiorespiratory fitness among adults, fewer studies have investigated the role of HIIT in children and adolescents, particularly in the context of the comprehensive obesity treatment using a rigorous study design. As a result, this application proposes to conduct a pilot randomized controlled trial to determine the feasibility and preliminary efficacy of integrating two distinct approaches to exercise training within an established lifestyle intervention for adolescent obesity. Specifically, 56 adolescents, age 12-16 years, with obesity participating in a 12-week lifestyle intervention will be randomly assigned to participation in one of two exercise training session conditions: 1) high-intensity interval training (HIIT) or 2) continuous, moderate-intensity training (CMIT). Both exercise programs will consist of three supervised sessions per week. In addition, all adolescents will participate in weekly behavioral weight management sessions that target specific dietary goals and promote self-monitoring behaviors. Assessments will be completed at baseline, post-intervention (12 weeks), and 1-month follow-up (16 weeks). Study results will be used to 1) determine the feasibility and acceptability of HIIT and CMIT as part of a comprehensive lifestyle intervention for adolescents with obesity; 2) investigate the preliminary efficacy of HIIT, compared with energy-matched CMIT, on adolescent adiposity, cardiorespiratory fitness and cardiometabolic risk factors; and, 3) explore if exercise training conditions (HIIT or CMIT) influence short-term PA levels and weight maintenance after completion of the lifestyle intervention. Results of these preliminary investigations will inform a fully powered trial examining the role of HIIT in the comprehensive treatment of adolescent obesity.