Abstract: Many biochemical,physiological and behavioural processes in organisms ranging from microorganisms to vertebrates exhibit circadian rhythms driven by endogenous oscillators.Since the cloning of the biological clock gene period in Drosophila,the endeavor in two decades to understand the molecular components that constitute these oscillators has been rewarded:we now know the circadian rhythm is driven by four cogs:PER,TIM,CLOCK and BMAL1,and the mechanism which govern the circadian oscillation has been elucidated.This work on biological clock genes is spectacular not only for its heuristic value,but also because it illustrate the synegistic power of genetics and genomics.Given the evolutionary conservation of the mechanisms that govern the fundermental biological processes,the period story will greatly accelerate the reseach progress on human complex behavior.