The field of chronobiology, the study of the rhythms in plants and animals, was restricted to botanists for centuries. Only recently during the last decades could research be broadened to include animals and later even human beings. Rhythms have been documented and related to the alternation of day and night and to the succession of the seasons. Nowadays, chronobiology has developed into a multidisciplinary field in which scientists are involved in basic research as well as in applied topics. This paper gives an introduction to the field, especially dealing with the aspect of rhythm development, and the way in which the different 24-hour rhythms in children become apparent.