The simple aim of this thesis is to discuss the evidence that associates serotonin, serotonin receptors implicated in the pathophysiology of depression and the serotonin transporter (SERT) to the etiology of depression and the mechanisms that underly the response to antidepressant treatment. Overall I would like to highlight the role of the serotonergic system in its entirety the mechanisms in which antidepressant medications, particularly the ones targeting serotonin, alleviate the symptoms of major depressive disorder.