Light is used by plants as a signal for many physiological and developmental processes. Phytochrome is the most extensively studied family of photoreceptors that plants use to perceive the presence and quality of light in their environment. While the initial action of the phytochrome molecule is not yet known, one important kind of response, changes in the expression of specific nuclear genes, has been intensively investigated. Although phytochrome-regulated promoters are complex and can also respond to other signals, specific DNA elements that are involved in conferring phytochrome responsiveness have been identified. Potential signal transduction pathway components include G proteins, cyclic GMP and Ca2+/calmodulin. In addition, the study of transcription factors involved in phytochrome-regulated gene expression has yielded insights into some of the final steps of transcriptional regulation by phytochrome.