ear Colleagues,The concept of chronobiology combines the notion of rhythms with objective phe-nomena reflecting the functioning of the living organism. Rhythms give a framework tothis functioning and are of great importance to our everyday life. Indeed, rhythms arepresent due to night and daylight cycles, meal periodicity, and social interactions, andeven in the work place. All these synchronizersfor which the German wordZeitgeber is often used, as a result of Jürgen Aschoffs seminal researchleave animprint on our lives.There are endogenous rhythms that correspond to these exogenousrhythms, such as sleep-wake cycles, rhythms in hormonal secretions, and other biologi-cal rhythms in general.In pathophysiology, some rhythms acquire an abnormal character, and some dis-orders exhibit specific rhythms. Examples include recurring episodes of manic-depres-sive illness, schizoaffective psychoses, and recurrent depression.The understanding of this chronological symptomatology and its correlation withchronobiology is essential for two reasons. First, clinically or biologically suitable mark-ers must be defined, and, second, treatments stimulating or regulating rhythms must bedevised. For instance, rhythms may be stimulated by antidepressant drugs in depression,or regulated by chronobiotic substances, such as mood-regulating drugs.We are convinced of the importance of a progress report on the current state ofthe art in these various fields, and we believe that the articles in this issue will provideplenty of food for thought.Yours sincerely,Jean-Paul Macher, MDMarc-Antoine Crocq, MDE d i t o r i a lD3 0 9