Many physiological phenomena are rhythmic and coincide with a particular time of day. These 'circadian rhythms' are not dependent on external timing cues but are driven by internal circadian clocks that are ubiquitous features of living organisms. Although many of these rhythms manifest themselves as complex behavioural patterns, we now know that a circadian clock does not require a complex organism or an elaborate nervous system; it can be built from molecules within an individual cell. This review focuses on new advances in identifying and understanding the basic properties of cellular circadian clocks.