Light is an essential part of a plant's environment, regulating many characteristics throughout its life. Plants have evolved several groups of photoreceptors to monitor the ambient light around them. Among these receptors, phytochromes (Phys) are dimeric red/far-red photosensing proteins, which were found more than fifty years ago. In more recent times, researchers have discovered new members of the phytochrome superfamily in eubacterial, cyanobacterial and fungal species, along with some Phy-like photoreceptors which function through a similar, yet modified mechanism. In general, Phys are composed of an N-terminal and a C-terminal fragment. The N-terminal chromophore-binding domain (CBD) is made up of a Per/Arnt/Sim (PAS) domain, a cGMP phosphodiesterase/adenyl cyclase/FhlA (GAF) domain and a Phytochrome-specific (PHY) domain. The C-terminus is either a histidine kinase (HK) domain or histidine kinase related domain (HKRD). The N-terminal light-sensing CBD is the main light switch that enables the detection of red (R) and far-red light (FR) by photoconverting between two stable conformers, Pr and Pfr. Pr is the ground state while Pfr is the active state which usually activates downstream signal pathways. Due to the complexity of phytochrome itself and its signaling pathways, most of the central issues in understanding how phytochromes work still remain unclear.