A two-arm randomized controlled trial (RCT) will be conducted to test a mobile-based intervention, E-breathing, for early detection and intervention to address psychological distress in at-risk adolescents in Hong Kong. E-breathing harnesses the science of real-time, in-situ assessment approach - Ecological Momentary Assessment (EMA) - to capture the psychological symptoms and needs of adolescents. Based on the real-time data collected via smartphone, the mobile application prompts the practice of slow-paced breathing as an Ecological Momentary Intervention (EMI) to reduce psychological distress. This RCT will recruit a total of 100 participants who will be randomly allocated to either (1) an intervention group (EMI group), where they will receive a feedback prompt when they score high on negative affect (i.e., E-breathing) or (2) the control group (EMA group), where they will only complete EMAs without feedback. This project aims to investigate the feasibility and effectiveness of E-breathing in improving psychological symptoms in Hong Kong adolescents. This project also examine the underlying physio-mechanisms of the effects of E-breathing on psychological outcomes in this target population.