Plant development is dependent on not only endogenous conditions but also environmental factors. One of the best examples of environmental regulation of plant development is photoperiodic flowering, by which plant flower in response to changes of day length ([Garner and Allard 1920]). The predictability conferred by the seasonal changes in photoperiod enables plants to flower at the most favorable time of the year. The question of what photoreceptors mediate photoperiodic flowering has been one of the focuses in our efforts to understand the underlying mechanisms of photoperiodism. An action spectrum for the photoperiodic regulation of flowering time was reported in as early as 1945, which showed that red light was the most effective spectrum of light used in the nightbreak experiments to inhibit flowering of SD plants, suggesting a red lightabsorbing pigment in the photoperiodic response ([Parker et al 1945]). It was later found that the red light effect could be reversed by far-red light which, together with a similar effect of light on germination, contributed to the discovery of phytochrome ([Borthwick et al 1952]). In addition to red light, blue/UV-A light has also been found to affect flowering time in some of the early works, but most of these light effects were attributed to phytochromes ([Parker et al 1946], [Meijer 1959], [Brown and Klein 1971]).We now know that, in addition to phytochromes, blue/UV-A light receptors also play important roles in the light regulation of flowering time ([Guo et al 1998], [Imaizumi et al 2003]). In the last 5 years, signifi- cant progress has been made in the study of plant photoreceptors and the molecular mechanisms underlying light regulation of flowering time.