Although treatment guidelines manifest that antidepressant response usually appear with a delay of several weeks and suggest that treatment should be changed if a partial response has not occurred after 4~6 week, these beliefs are no longer held by experts, and a new concept is raised that the first 2 weeks of treatment may be a useful strategy for improving the management of depression. New evidence indicates that early treatment response can be predicted with high sensitivity after 2 weeks of treatment in patients with major depressive disorder (MDD).