Photoperiodism is the physiological reaction of plants to the relative lengths of light and dark periods, which affects their growth, development, and particularly flowering. The phenomenon is based on the ability of plants to measure the duration of day and night, and it is crucial for synchronizing reproductive processes with favorable environmental conditions. The principles of photoperiodism are largely rooted in the perception of light, the measurement of day length, and the hormonal regulation of flowering. A central principle of photoperiodism is that plants respond not to the absolute amount of light but to the duration of uninterrupted darkness. This dark period is sensed by a specialized photoreceptor system that includes phytochromes, which are light-sensitive pigments that exist in two interconvertible forms: Pr, which absorbs red light, and Pfr, which absorbs far-red