Publisher Summary This chapter discusses photoperiodic photoreceptors in plants. The detection of light for photoperiodic responses in angiosperms is primarily accomplished by members of the phytochrome family of photoreceptors, with the exception of members of the Cruciferae where a blue-light photoreceptor also plays a photoperiodic role. Lower plants, on the other hand, primarily use blue-absorbing photoreceptors for photoperiodic perception. Much of the physiological evidence for phytochrome involvement is based on action spectroscopy of night-break responses, where very similar action maxima are found in the red for the inhibition of flowering in SDP and the promotion of flowering in LDP. Reversal of the effect of R in inhibiting flowering by FR is diagnostic for regulation by phytochrome. However, an extended family of phytochrome genes, encoding a number of distinct regulatory proteins with R/FR properties, has recently been revealed. Mutants lacking phytochrome A or phytochrome B have shown that these two phytochromes have distinct, although possibly overlapping, functions as far as mediating the inhibition of extension growth by light of different wavelengths is concerned. By extrapolation, it may be expected that different members of the phytochrome family will have different physiological roles and some of these will include light detection for photoperiodic responses.