The first level group will be treated with a single SSRI antidepressant treatment. The second and third level groups, who received SSRI treatment before, will be treated with alternative SSRI antidepressants (switching), combined multiple SSRI treatments (antidepressant combination), or SSRI treatments combined with mood stabilizer or anti-psychotics (augmentation). This study does not use placebos. Patients will visit 5 times for 6 weeks at each level for treatments. Patients will be evaluated for the severity of depressive symptoms, functional level, and side effects at each visit. Afterwards, the investigations will combine to monitor the patients depressive symptoms in every 3 months for the next 24 months. 18 nationwide university hospitals will participate in this study. This multi-site, prospective, and naturalistic study for patients with depression in Korea is a main project of 'Clinical Research Center for Depression' funded by the Ministry for Health, Welfare, and Family Affairs (MIHWAF) in Korea for a highly-qualified research achievement.