Abstract. Over several decades, formal experiments measuring diapause responses to variable light inputs have indicated that photoperiodic time measurement in insects is accomplished either by a nonoscillatory ‘hourglass-like’ mechanism or by oscillatory components of the circadian system. Although both are possible given the present state of our knowledge, a substantial body of evidence strongly suggests that night-length measurement is a function of the circadian system, and that ‘hourglass-like’ clocks are manifestations of damping circadian components. The two types of time measurement, ‘hourglass’ and circadian, are therefore parts of a spectrum of mechanisms differing in their damping coefficients. If this view is correct, it may follow that genes and proteins involved in circadian timing are also involved in photoperiodism, although additional genes, or known ‘clock’ genes used in novel ways, may also play a part. This review outlines the experimental evidence for the oscillator clock theory of photoperiodism and suggests ways in which further progress may be made.