Plants, as many other organisms, synchronize the timing of their physiology and development by using an endogenous mechanism called circadian clock. Perception of environmental changes during the day-night cycle is crucial for circadian function, which relies on transcriptional feedback loops at the core of a central oscillator. Recent studies in Arabidopsis have shown that the transcriptional regulation of clock gene expression is governed by rhythmic changes in chromatin structure. The chromatin remodelling activities relevant for clock function are modulated by day-length or photoperiod, suggesting a mechanism by which the plant clock synchronizes development with the external time. Evidence that a central component of the mammalian clock has histone acetyltransferase activity suggests that chromatin remodelling has evolved as an important mechanism for circadian function.