Light signals are among the most important environmental cues regulating plant development. Three small families of chromoproteins are currently known to function as sensory photoreceptors in plants: the red/far-red (R/FR) light-sensing phytochromes and the UV-A/blue light-sensing cryptochromes and phototropins. Substantial progress has been made in defining the structures and photochemistry of these receptors, their physiological and molecular functions, and their cellular transduction mechanisms. Five phytochromes, phyA–phyE, are present in Arabidopsis and PHY gene families encoding proteins similar to these are found in all flowering plants. Moreover, it appears likely that heterodimerization of phytochrome proteins generates an even more extensive and diverse array of these molecules. Our growing understanding of phytochrome signaling mechanisms derives most recently from genetic analyses of PHY gene mutants and other photomorphogenic loci, characterization of the expression patterns and light-regulated cytoplasmic/nuclear distributions of the phytochromes, and genomic approaches to identifying phytochrome binding partners, components of their signaling networks, and the target genes of these networks. Such studies have provided an increasingly complicated, yet still incomplete, picture of how absorption of R and FR photons by phytochrome molecules triggers cellular events that endow plants with the capacity to monitor and adapt to a complex and changing natural light environment.