This study is a 6-week open-label clinical trial in which Seroquel XR is added to an antidepressant medication for the treatment of MDD with or without comorbid generalized anxiety disorder. Patients who are currently experiencing a depressive episode that has not responded to an adequate trial of an antidepressant will be eligible to participate. During the course of augmentation treatment with Seroquel XR, each subject will regularly enter information on symptom severity and functional impairment into their handheld computer, and this information will be wirelessly transmitted to a central database.