The circadian system maintains daily rhythms in many physiological processes in higher plants, ranging from growth to gene expression. Circadian timing is intimately connected to light signalling. Light-regulated elements are interwoven with the promoter sequences required for the circadian regulation of clock-controlled genes. Phototransduction also has a major influence on the period of circadian rhythms in plants, and may be required for the oscillator mechanism. Our understanding of the oscillator in plants is moving towards some major advances: newly-discovered rhythm mutants suggests closer links between light signalling, the circadian oscillator and transcriptional regulation. (C) 1998 Annals of Botany Company.