Current major depressive episode of at least 4 weeks duration and for the current episode of major depression has a history of failure to respond to at least 4 weeks of any antidepressant medication such as SSRI, Wellbutrin, nefazodone/trazadone, MAOI, or venlafaxine/duloxetine at an adequate or clinically customary dose for antidepressant purposes prior to entering escitalopram screening. Informed consent must be obtained in writing for all subjects at enrollment into the study.