Drosophila melanogaster is an ideal model system for understanding the molecular mechanisms of circadian rhythm. The genetic amenability of Drosophila has led to the identification of more than ten clock genes and a set of clock-related genes, including clock input and clock-regulated genes. These clock genes and their products consist of two interlocked transcriptional/translational feedback loop, which regulate circadian rhythms of behavior and physiology in Drosophila. The working principles of the Drosophila core clock are also seen in mammals.