The circadian clock keeps time through an intracellular oscillator that requires rhythmic gene expression. In Drosophila melanogaster, the core of this oscillator is composed of a circadian feedback loop in which the transcription of the period and timeless genes is repressed by their own protein products. In the past year, our understanding of clock organization and function in Drosophila has been advanced by breakthroughs that define when, where and how this feedback loop operates. These studies, along with those in other organisms, suggest that circadian feedback loops are widespread and that genes within these feedback loops are conserved between Drosophila and mammals.