Publisher Summary This chapter focuses on the photoperiodic timekeeping in plants. In order to locate the time of year accurately, the timekeeping mechanism in photo-periodism must operate with a considerable degree of precision. It must also be relatively insensitive to random variations in the environment. This applies particularly to changes in temperature, or to light quality where this is due to factors such as sunflecks passing over a woodland floor. Because the rate of change of day length is not constant but is fastest in spring and autumn, the absolute precision will vary with the time of year. The rate of change is lower in the tropics during much of the year and timekeeping needs to be more precise than at higher latitudes in order to locate a seasonal event with the same degree of accuracy. Organisms are normally subjected to daily alternations of light and darkness and often exhibit rhythmic behaviour in association with these changes. Many of the rhythmic responses to day and night continue even in a constant light or dark environment, at least for a period of time.