The lives of plants, animals, and human beings are all regulated by circadian clocks. In mammals, 24-hour rhythms of physiology and behavior are directed by a master clock in the suprachiasmatic nucleus (SCN) of the brain hypothalamus, which in turn entrains "slave" oscillators of similar molecular composition in most cells of the body. These peripheral clocks are interesting not only because they control many aspects of circadian physiology, but also because they are model systems through which we understand how the SCN regulates complex behavior. To this end, peripheral oscillators have been exploited both biochemically to understand the proteins that make up biological clocks, and genetically to decipher the ways in which individual differences in human chronotype might arise.