1. The present brief review examines some of the new developments in the area of circadian rhythm research. 2. The discovery of the mouse clock and m-per genes and their similarity to other clock genes like per and tim has provided new insight into the control of rhythms in vertebrates. In mice, these genes are expressed in the site of the biological clock, the suprachiasmatic nucleus (SCN), and so will now become a focus of research into the generation of rhythmicity. 3. Because SCN cells expressing endogenous rhythms have a periodicity different from 24 h, there must be mechanisms in place to reset the rhythms on a daily basis. This is achieved in mammals by retinal light perception and neural transmission through several discrete pathways to the SCN. 4. The nature of the neurotransmitters involved in this transfer of environmental information to the timing system is controversial and may even very between similar species but, in the rat, there is compelling evidence that a serotonergic pathway is pre-eminent in mediating the effects of light. How the re-setting is achieved at the cellular level is not known.