Social anxiety disorder (SAD) is among the most common psychiatric conditions and is associated with significant distress and dysfunction in social situations. Although treatment with cognitive-behavior therapy (CBT) is known to help remedy SAD, many patients do not respond to this treatment and most do not reach full recovery. In CBT, patients undergo repeated and prolonged exposure practices to feared social situations to learn better ways to deal with anxiety in these settings. Exposure therapy is based on animal models of extinction of conditioned fears, and recent animal research has identified some of the core pathways and neurotransmitters involved in fear extinction. D-cycloserine (DCS) is a drug that appears to facilitate learning and the process of extinction of conditioned fear in both animals and humans. This study will assess the effectiveness of DCS combined with CBT in treating people with SAD.