In the fruit-fly Drosophila, rhythmic expression of the clock gene period is detected in cells throughout the body. Whereas these cells could be pacemakers for circadian rhythms of unknown physiological processes, the brain pacemakers are known to be responsible for circadian behavior. Recent progress in genetic and molecular studies of clock genes in Drosophila has permitted the identification of brain pacemakers at the cellular level and their output pathways to rhythmic behavior. Similar studies in other insect species have suggested considerable diversity in the anatomical and neurochemical properties of pacemaker cells, as well as in the mechanisms of clock-gene regulation.