 Social anxiety disorder SAD is characterized by intense and debilitating anxiety in common social situations. Cognitive behavioral therapy CBT is a talking treatment that aims to reduce immediate anxiety symptoms. However, some anxiety symptoms and comorbid disorders not directly addressed by CBT may only improve in the short term, while recurring in the long term. Behavioral therapy based on acceptance and mindfulness is thought to have longer lasting effects, because this approach emphasizes accepting anxiety instead of controlling anxiety. This study will compare standard CBT to acceptance and commitment therapy ACT, which uses acceptance and mindfulness, to determine which is more effective on both a short- and long-term basis. Participants with SAD will undergo brain scans to determine how the two therapies affect brain functioning. 