Both serotonin and the serotonin transporter, which transports the neurotransmitter serotonin from synaptic spaces into presynaptic neurons, play an important role in the pathophysiology of several psychiatric disorders. Mutations associated with the serotonin transporter gene may result in changes in serotonin transporter function. The serotonin transporter gene promoter variant, consisting of a long (L) and a short (S) variant, is one of the major factors which contribute to the etiology of many psychiatric disorders. In this regard, many studies have been published on association of this variant with various psychiatric disorders. This repeat length variant in the promoter region of this gene has been shown to affect the rate of serotonin uptake and may play a role in post-traumatic stress disorder and depression-susceptibility in people experiencing emotional trauma. Associations between a functional variant in the serotonin transporter anxiety-related personality traits were found, as well as the risk of developing depression, alcoholism or suicidal behavior. Understanding of possible associations of these variants and psychiatric disorders would bring progress in principles and treatment of many disorders.