The book shows that cell membranes vary according to the 24h cycle: it deals with circadian changes in membrane composition, principally the plasma membrane and with structural organization changes in some chloroplast thylakoids. The book deals with changes in activity or efficiency of pumps, channels, photo- and hormone receptors, in sensitivity towards external signals, in sensitivity to some drugs, including anaesthetics and in changes in signal transduction. The cell cycle is discussed on theoretical and experimental grounds, as well as its gating by circadian rhythmicity. The rhythm generating mechanism is modelized. The circadian oscillation of the plasma membrane confers a temporal parameter.