Serotonin (5-hydroxytryptamine, 5-HT) has long been suspected to play a role in the etiology of depression, and modern neurochemical techniques have confirmed this suspicion. Furthermore, all drugs known to be selective (a relative term) serotonin transporter (SERT) inhibitors are effective antidepressants. Of the selective serotonin reuptake inhibitors (SSRIs) approved in a number of countries for use in depression, panic disorder, and obsessive-compulsive disorder, citalopram is the most selective. Citalopram has been used worldwide to treat an estimated 35 million patients, with an excellent safety record. Citalopram is a racemic drug, and its effects on serotonin transport are thought to reside in the S-enantiomer, known as (S)-citalopram or escitalopram. Escitalopram is the most selective SSRI yet developed. Its receptor binding properties and activity in preclinical animal models of depression predict that escitalopram would be effective in the treatment of depression, with approximately twice the potency of the racemate. The pivotal clinical trials of escitalopram not only support this conclusion, but also suggest escitalopram possesses advantages over citalopram in terms of both efficacy and safety. In conclusion, escitalopram is a promising candidate for use as a first-line antidepressant.