The discovery of clock genes and the general principle of their oscillation has stimulated research on biological clocks and this research has had a major impact on the field of life sciences. The mammalian circadian core oscillator is thought to be composed of an autoregulatory transcription-(post)translation-based feedback loop involving a set of clock genes. The real time monitoring of clock gene oscillation at the levels of genes, cells, tissues, and systems will clarify the issue of how the time signal is born and how it is integrated into the organismic level. Investigations of circadian systems in various organisms employ multiple methods including ethology, physiology, neuroscience, molecular biology, cell biology and genetics. The circadian system has thus become a unique example in the elucidation of the general principles of how genes control cellular, systemic and behavioral functions.