The ability to anticipate the day-night cycle and direct physiology accordingly has proven to be a general phenomenon across all kingdoms of life. Considerable fitness benefits are conferred by an internal 24-hour clock, which is known as a circadian clock. Extensive multi-disciplinary studies in a range of model organisms have elucidated many of the components involved in generating and sustaining daily rhythms. When comparing the circadian systems across the kingdoms, it is fascinating to observe the commonalities and differences in their molecular architecture, and the many adaptations which have evolved to deal with organism-specific requirements of biological timing.