To be prepared for the periodic alterations of light and darkness in their environment, plants utilise an endogenous clock to anticipate these changes and to time physiological processes appropriately. Large-scale screens for mutants with perturbed circadian output rhythms, as well as characterisation of mutants with altered flowering time in Arabidopsis thaliana (L.) Heynh., have led to advances in our knowledge of the molecular basis of this internal timing system. The repertoire of proteins, mostly nuclear-localised, that play a crucial role in clock regulation, as well as of the photoreceptors involved in transmitting light information to synchronise the endogenous clock with the outside world, has expanded considerably and is the subject of this review.