Reactivity to perceived social threats is a normative response that has considerable adaptive value to establish one’s position in a social hierarchy, but also routinely goes awry when perceived threats are exaggerated. For example, social anxiety disorder (SAD) is one of the most prevalent mental disorders, affecting 12.1% of the U.S. population at some point in their lifetime, resulting in serious impairment and a poorer quality of life. Yet, individuals with SAD often wait more than a decade before seeking treatment, if they ever do seek care, given that social avoidance is a core feature of social anxiety. This makes seeking in-person services very difficult, so there is a crucial need for scalable, accessible treatments that are delivered outside of therapy and can be integrated into daily life. Just-in-Time Adaptive Interventions (JITAIs) delivered via smartphones represent a promising method to not only increase access to cost- effective and acceptable mental health care, but also to tailor in-the-moment interventions to best match the specific context of the individual and their personal stressors and determine when the individual is most likely to benefit from the intervention. This work leverages mobile sensing to detect indicators of temporal phases of social anxiety and personal context to optimize treatment. This project proposes the Context- Aware Micro-Interventions for Social Anxiety (CAMSA) system, targeted at understanding relevant contexts of social anxiety and delivering personalized interventions to reduce symptoms. This 3-phase project will overcome fundamental barriers to continuous and accessible treatment. First, the CAMSA system will be developed, consisting of sensor-rich smartphones and smartwatches that will collect data on relevant features (e.g., type of anxiety, social context, physiological state) that identify an individual's state anxiety context, along with user-centered design of personalized micro-interventions for social anxiety. Second, the CAMSA system will be deployed to socially anxious individuals to identify biomarkers of state anxiety at different temporal phases and optimize features to personalize the content and timing of the intervention delivery. Third, a proof-of-concept for CAMSA will be demonstrated through a pilot study, with anxious participants receiving JITAIs. RELEVANCE (See instructions): Social anxiety disorder (SAD) is one of the most prevalent mental disorders, affecting 12.1% of U.S. individuals at some point in their lifetime. SAD is characterized by fear and avoidance of socially evaluative situations, often leading to devastating consequences for the affected individual and placing considerable economic burden on society at large.