Additional Study Information: This research study uses brain imaging to explore the biological causes of major depression and why some people feel suicidal or may act on such thoughts. We hope to develop a better understanding of how people who feel depressed and suicidal cope with their feelings so we can improve treatment and prevention. Participants can have depression with or without suicidal thoughts or any history of suicide attempts. Participation involves a detailed diagnostic set of interviews, brain imaging scans (MRI and PET) and neuropsychological tasks. Eligible individuals will be compensated up to $475 and may receive up to 6 months of free doctor visits for medication-based treatment.