The suprachiasmatic nucleus (SCN) is considered the master circadian pacemaker that controls most of the physical circadian rhythms of mammals including behavior. Studies of clock genes have implied that oscillatory mechanisms function in peripheral organs and isolated cells, and that they are entrained to the SCN. The circadian clock is an intracellular, transcriptional-translational mechanism that shares the same molecular components in the SCN and in peripheral cells. Hundreds of tissue-specific circadian genes that regulate an impressive diversity of biological processes have been identified using DNA microarray technology. This review summarized the recent information of clock and clock-controlled genes in mammals.