Circadian oscillators are time-keeping mechanisms that cells use to maintain a 24 hour rhythm and drive periodic expression of genes controlling physiology, metabolism and behaviour. In each oscillator, two interlocked transcriptional and translational feedback loops ensure that the mRNA levels of transcription factors such as Clock (CLK) oscillate out of phase with those of proteins such as Period (PER) that oppose their effects. In a Commentary onp. 3369, Nicholas Glossop and Paul Hardin compare the central `master' clock in the brain with peripheral clocks present in other tissues. Recent work indicates that the central oscillators differ significantly from their peripheral counterparts —for example, in the fly, peripheral oscillators require the blue-light-responsive protein cryptochrome (CRY) whereas the central oscillator does not. Furthermore, studies such as analyses of clock component mRNA cycling have revealed that peripheral oscillators from different species are strikingly similar. On the basis of these observations, Glossop and Hardin argue that the peripheral oscillators from different species reflect a basic clock design, from which central oscillators subsequently diverged.