This work is a guidebook for clinicians who are involved in treating depressive patients and also serves the research scientists who are working on the psychopharmacological mechanisms of antidepressant actions and psychopathological mechanisms underlying mood disorders. Mood disorders such as major depressive disorder (MDD), bipolar disorder (BPD) and seasonal affective disorder (SAD) are the most disabling disorders that are among the most expensive of all medical illnesses. The pathophysiology of mood disorders is very complex and involves many mechanisms like circadian rhythm disruption, sleep abnormalities, melatonin rhythm abnormalities and alterations in melatonin receptor mechanisms, abnormalities   in monoaminergic neurotransmitter mechanisms, glutamatergic release mechanisms, hippocampal neurogenesis, and abnormal immune and cytokine release mechanisms.