Adolescence is a period of heightened vulnerability for mental health problems, particularly for the onset of anxiety and depression. Adolescent-onset anxiety and depression can lead to a wide range of negative consequences across the life-course, including elevated risk of recurrent episodes of anxiety and depression in adulthood, and poorer overall functioning. Stressful life events are well-known risk factors for anxiety and depression during adolescence, though the mechanisms linking stressful life events to the onset of youth anxiety and depression are not well described. Understanding mechanisms of stress vulnerability in adolescence will provide valuable targets for preventing anxiety and depression during this key developmental window of risk. The current study involves regular monthly assessments, which allows us to intensively examine ‘real time’ changes in emotion, behavior, physiology, and brain networks underlying emotional processing following stressful life events. This study requires participants to complete monthly MRI scans, surveys and interviews that measure levels of stress and mental health, and install applications onto their phones to track physiological activity, social behavior and self-reported wellbeing.