ear Colleagues,Life on earth developed within the constraints of order and chaos, in the sense thatphysiology oscillates between situations of predictable regularity and less predictable appar-ent randomness.These behaviors reflect the influence of synchronizing factors, some exter-nal to life, others inbuilt within the organisms themselves.Chronobiology, defined as the study of rhythms in living species, is a science withmultiple aspects, from molecular biology and biochemistry to clinical phenomena andobservations.Among the objectives of chronobiology research, finding the links that exist betweenfundamental and clinical aspects is relevant to human medicine. Indeed, linking biologicalmarkers of altered endogenous rhythms to the manifestations of disorders is part of theongoing effort of several research groups.This important research objective is leading usto the discovery of chronobiological markers for the purpose of diagnosis, as well as to abiologically based validation of clinical categories of disorders, a necessity for the future ofpsychiatry practice. Findings from research in chronobiology will also be determinant inthe invention of new therapeutics aimed at restoring normal endogenous rhythms.Psychiatric disorders are known to have more or less rhythmic patterns of manifes-tation, as in the case of mood disorders, and other disorders such as several psychotic dis-orders, seasonal affective disorder, and premenstrual dysphoric disorder.They are also char-acterized by spontaneous or induced changes in endogenous rhythms, as in the case of sleepdisorders or neuroendocrine changes.This issue on chronobiology of Dialogues in Clinical Neuroscience aims at giving thereader an overall view on the field of chronobiology, from articles within the field of mol-ecular research on biological clocks, to findings about clinical techniques aimed at the sta-bilization of abnormal biological rhythms.The authors have succeeded in presenting complex information in a didactic man-ner, and we wish to express our gratitude for their work.We hope that their contributionswill help with the diffusion of the idea that a practice of psychiatry integrating chronobiol-ogy is probably inevitable and potentially fruitful.Sincerely yours,Jean-Paul Macher, MDE d i t o r i a lD