Recent research on the cellular basis of circadian rhythmicity has stressed the importance of clock genes for the maintenance of normal rhythmicity. There have been tremendous advances in our understanding of the inter-relationships of the various genes known to generate the rhythms. We know relatively little, however, about the way animals maintain their rhythmicity under normal seasonally changing photoperiods, from the level of neurotransmitters involved in the transfer of photic information to the way the clock genes respond. In this short review, some aspects of entrainment are discussed concentrating on species differences in transmitters used, particularly the role of serotonin. Also the effects of different experimental paradigms on outcomes, for example phase response curves and skeleton photoperiods, are discussed.