Circadian oscillators are cell-autonomous time-keeping mechanisms that reside in diverse tissues in many organisms. In flies and mice, the core molecular components that sustain these oscillators are highly conserved, but the functions of some of these components appear to have diverged significantly. One possible reason for these differences is that previous comparisons have focused primarily on the central oscillator of the mouse and peripheral oscillators in flies. Recent research on mouse and Drosophila peripheral oscillators shows that the function of the core components between these organisms may be more highly conserved than was first believed, indicating the following: (1) that central and peripheral oscillators in flies do not necessarily have the same molecular mechanisms; (2) that mammalian central oscillators are regulated differently from peripheral oscillators; and (3) that different peripheral oscillators within and across species show striking similarities. The core feedback loop in peripheral oscillators might therefore be functionally well conserved, and central oscillators could be specialized versions of a basic oscillator design.