ear Colleagues,The known properties of the central nervous system are quite remarkable and wemay confidently assume that many more fascinating aspects of the brain remain to bediscovered. Until recently, the causation of mental disorders was always explained interms of abnormalities involving familiar biological concepts, such as monoamine neu-rotransmission, receptor regulation, and molecular biology.The appearance of a novel explanatory model, accounting for some previouslyunexplained phenomena, is of tremendous interest. It has long been known that somedisorders involve regional modifications that can be evidenced by studying brain struc-ture. Neurotrophic factors preventing cell death have been shown to exist and, morerecently, the process of hippocampal neurogenesis has been described.Neuroplasticity is the process that underlies neurogenesis: it leads to protein syn-thesis and constitutes a defense mechanism against the deleterious effects of stress. Plasticmodifications of neurons and synapses have been observed thanks to the developmentof neuroimaging techniques, which can reach as far as the cellular level.The observationsrelating to neuroplasticity have led to: New diagnostic markers. A better understanding of certain pathogenetic mechanisms. The proof of activity of certain compounds.We believe that it is important to give a progress report on the concept of neuro-plasticity and its influence on the understanding of the mechanisms of depression. Weare grateful to Dr David R. Rubinow from the National Institute of Mental Health inBethesda, Md, for bringing together the most qualified authors in the field to discuss thistopic in this issue of Dialogues in Clinical Neuroscience.Yours sincerely,Jean-Paul Macher, MDMarc-Antoine Crocq, MDE d i t o r i a lD1 1 3