The pathophysiology of major affective illness is poorly understood. However, several lines of preclinical and clinical evidence indicate that an enhancement of 5-HT-mediated neurotransmission might underlie the therapeutic effect of most antidepressant treatments. This net effect would, however, be obtained via different mechanisms. A better understanding of the neurobiological basis for the delayed onset of action of antidepressant treatments has led to the elaboration of strategies that could accelerate the antidepressant response. These strategies are discussed in this article by Pierre Blier and Claude de Montigny.