Extraordinary progress has recently been made towards the understanding of how the plant photoreceptor phytochrome is able to transduce light signals into biological responses. This has been achieved by a combination of complementary genetic, biochemical, and cell biological approaches. We can now assign individual phytochromes to particular responses, and have knowledge about the nature and workings of some of the signal transduction intermediates utilized. This review summarizes our current knowledge of phytochrome phototransduction mechanisms.