It is an established fact that circadian rhythmicity is often somehow involved in the physiology of photoperiodic induction. It is shown, however, that there are three possible ways in which such rhythmicity could be involved. For the most part available data are inadequate to discriminate among these three roles, only one of which is covered by "Bünning's Hypothesis." Emphasis is given to a previously ignored possibility in which circadian organization is involved in photoperiodism"ut not as the clock responsible for the time-measurement. The meaning of circadian surfaces and their bearing on the interpretation of a widely used experimental protocol is developed.