Published research studies on the antidepressant activity of ketamine in the last twenty years have significantly changed the way people think about potential new antidepressants and the biological basis of depression. The symptoms of depression may subside for several days after the administration of a dose of ketamine. In contrast, achieving a therapeutic effect with classic antidepressants requires chronic administration. The critical issue for ketamine is understanding the biological basis of its amazing effects. Because one of the main molecular mechanisms of ketamine action is the blockade of NMDA-activated glutamate receptors, a great effort has been directed at understanding the role of the glutamate system in the pathophysiology of depression and the unique antidepressant profile of ketamine. This review discusses the most relevant glutamate hypotheses explaining the molecular and cellular mechanisms of ketamine action. In the first place, phenomena such as the disinhibition of glutamate release and the inhibition of NMDA receptors stimulated by spontaneously released glutamate are discussed, followed by the relationship between the antidepressant effects of ketamine, glutamate, and the functioning of the lateral habenula. The last part of the review discusses the involvement of the individual enantiomers and metabolites of ketamine in its antidepressant activity.