Observing nature we realize the existence of rhythmic phenomena that cover time frames of different lengths. These range from short rhythms, such as that of breathing or heartbeat, to phenomena that last for hours, such as the sleep/wake cycles or the flower opening, up to events lasting months or years, such as hibernation or the mainteinance of prey/predator balance. There are other rhythms hidden from direct observation: examples are the rhythms of brain activity, the secretion of many hormones and the gene activity in the cells. This article is a brief (and by no means exhaustive!) introduction to chronobiology, or the branch of biology that studies periodic phenomena in biological systems.