Abstract: Many physiological and biochemical processes in plants exhibit endogenous rhythms with a period of approximately 24 h. These rhythms are regulated by the biological clock, which includes an input pathway, a central oscillator, an output pathway and a gateway. The light signal incorporated into the biological clock through phytochrome and cryptochrome has multiple influences on the central oscillator, inducing many clock-regulated processes through output signals. This paper reviews the signal transduction pathways regulated by light in the biological clock.