Light seems to be the only zeitgeber signal that reaches the pacemaker via specific pathways. For all nonphotic entraining agents, there might be a common mechanism that operates by feedback processes via "arousal." When entrained, the circadian system keeps a distinct phase relationship to the zeitgeber. This is achieved by daily corrections of frequency and/or phase. The phase response curves (PRCs) obtained with brief light- or dark-pulses are satisfactory in explaining such non-parametric entrainment. However, many organisms are exposed daily to one major step-up and one step-down in light intensity; hence, attention should be payed to the usefulness of step-PRCs. Zeitgebers not only entrain circadian rhythms by controlling the phase and period of the pacemaker, but also mask the overt rhythm. Many experimental data demonstrate a dependence on phase of the masking effects. It is due to this dependence on phase that organisms that become arrhythmic in constant conditions can show freerunning rhythms under the influence of permanent high-frequency light-dark (LD) cycles (the "demasking" effect). Under natural conditions, most animals are exposed to continuously fluctuating light intensities. As demonstrated by experiments with high-frequency LD cycles, either self-selected or externally controlled, the circadian system integrates over the intensities experienced, and changes its frequency accordingly. It seems likely that non-parametric entrainment plays a predominant role, but contributions of parametric effects are probably underrated.