Through the use of genetically amenable model systems, we have begun to form a relatively clear idea as to the molecular mechanisms that constitute a functioning circadian clock. It is now known that mechanisms that underlie overt rhythms are conserved across species. At the basic core of the clock lies a transcriptional/translational feedback loop. The primary components of this loop are called clock genes and are similar for the fruit fly, Drosophila melanogaster, and mammalian systems. However, many questions regarding their regulation remain unanswered. In addition to their localization in brain areas associated with pacemaking function, clock genes are also found in peripheral tissues where their presence may confer circadian regulation upon local, tissue-specific functions. The light-dark cycle is the primary environmental stimulus for the synchronization of the circadian clock. In Drosophila, light is known to induce the degradation of a clock component resulting in the synchronization of the core clock mechanism. Photic signals are transmitted to the clock, at least in part, by the blue light photoreceptor cryptochrome. Although expression of several mammalian clock gene products is also altered in response to light, the photoreceptor(s) involved have not yet been defined.