Authors: Larrazabal, M. A., Cai, L., Toner, E. R., Boukhechba, M., Barnes, L. E., & Teachman, B. A. (2021) Social anxiety disorder (SAD) is a highly prevalent and impairing condition that is undertreated due to structural barriers (e.g., cost) and social avoidance (a core feature of SAD). Technology-assisted interventions that can be delivered to individuals in their daily lives can increase treatment access. Successful deployment of these interventions requires detecting when individuals are feeling anxious. To this end, we developed a laboratory procedure to identify biomarkers of anxiety as it unfolds during social and non-social situations. Our procedure can be implemented in-person or virtually. It involves asking participants to watch a video (individual experience) and have conversations with one (dyadic experience) or five other participants (group experience) while passive sensors capture their heart rate, electrodermal activity, speech, facial expressions, and body movements. To add social threat, participants are told before some dyadic and group experiences that their performance will be evaluated by their conversation partner(s). We examine anxiety biomarkers before (anticipatory), during (concurrent), and after (post-event processing) the experiences. In a sample of 31 students reporting high or low social anxiety, results indicate that audio, body movements, and skin temperature help differentiate when individuals are alone vs. interacting with other(s). Additionally, audio features help detect when individuals are experiencing anticipatory, concurrent, or post-event processing anxiety within an interaction. These findings suggest that passive data can help identify when social anxiety is more likely (i.e., during social interactions) and the stage of the anxiety (e.g., anxiety about an upcoming vs. past event). Identifying these passive markers of anxiety is a first step toward unobtrusively detecting anxiety as it occurs in real time and offering individuals personalized interventions when and where they need them the most.