 Participation in this study will include one testing session, which will take between 3 and 3.5 hours. Participants who receive treatment for their anxiety may be asked to complete this testing a second time, after their treatment. During the testing session, sensors will be placed on participants skin in several locations to measure heart rate, sweat response, and eye-blinks. Participants will then be asked to respond to multiple stimuli: sounds, lights, smells, pictures that elicit different types of emotions, an interaction with an unfamiliar person, and specialized toys. In addition, participants will undergo blood testing and have several samples of their saliva collected on the day of the testing session. Participants will also be asked to collect additional saliva samples at home three times a day on 4 different days. Child participants and their parents may also be asked to complete questionnaires and interviews about behavioral and emotional problems. 