In recent years, there has been a flurry of activity directed towards identifying the molecular basis of circadian (approximately 24 h) rhythms. The past year has seen the isolation of the first clock mutations in a number of organisms (mice, Arabidopsis, cyanobacteria), the identification of a new circadian rhythm gene in Drosophila that interacts with the well known period gene, and considerable progress in the analysis of the 'clock genes', period and frequency. A combination of genetic, molecular and biochemical approaches is leading to an emerging picture of how molecular events enable organisms to keep time.