Just about anywhere you look in the natural sciences you'll find a cycle of some description. From cells to individuals to populations and beyond, cyclical patterns exist on every scale. The following collection of Web sites follows on this theme: The first site (1) is an excellent, animated introduction to the cell cycle from Cells Alive! Users can also get a closer look at the stages of mitosis by following the links provided. The next site from the Center for Biological Timing contains a tutorial on chronobiology, the study of biological rhythms (2). Visitors will find a thorough overview of the human clock and related concepts, with emphasis on our 24-hour sleep-wake cycle. Speaking of internal cycles, the next site contains an interesting article from BBC News, relating how a woman's choice of men may vary in accordance with her menstrual cycle (3). The next site moves from internal to externally-evident cycles, namely the life cycle of the monarch butterfly. Monarch Watch provides a detailed description of the butterfly life cycle, from egg to larva to pupa to adult (4). Perhaps not quite as appealing as the monarch butterfly, but nevertheless intriguing, Schistosoma flatworms have a complicated life cycle involving humans and a particular group of snails. The University of California-Los Angeles Institute of the Environment offers an illustrated explanation of this highly specialized life cycle (5). The following site from Science New Online describes how global climate change is accelerating the annual life cycles of plants and animals around the world (6). On the level of population, the Natural History Museum of Los Angeles County offers the familiar example of the Canada lynx and the hare -- the same example used by countless textbooks to demonstrate the cyclical nature of predator-prey dynamics (7). And finally, Georgia State University provides a nicely simplified introduction to the energy cycle in living things, focusing on the transfer of energy from the sun to plants to animals (8).