Circadian rhythms are regulated by clocks located in specific structures of the central nervous system, such as the suprachiasmatic nucleus (SCN) in mammals, and by peripheral oscillators present in various other tissues. Recent discoveries have elucidated the control of central and peripheral clocks by environmental signals. The major synchroniser in animals is light. In mammals, a subset of retinal ganglion cells receive light signals that are transmitted to the SCN via the retinohypothalamic tract. Photoreception is probably elicited by a novel opsin, melanopsin, although cryptochromes may also play a role. These signals feed directly to the SCN master clock, which then provides timing cues to peripheral clocks. In contrast to mammals, peripheral tissues in the fly and in the fish are directly photoreceptive. However, alternative routes exist. Some peripheral clocks in mammals can be specifically entrained in an SCN-independent manner by restricting food during the light period.