Cryptochromes compose the widespread class of blue-light sensory receptors that in plants regulate processes of development and circadian rhythm. These photoreceptors can also function as magnetoreceptors. Cryptochrome proteins bind flavin adenine dinucleotide (FAD) as a chromophore in the photolyase homology region (PHR) domain and contain the C-terminal extension (CCE) which is joined to PHR near the FAD-binding site. The cryptochrome activation is initiated by photochemical FAD conversions involving electron/proton transfer and the formation of redox forms. In plants, cryptochrome protein with photoreduced FAD undergoes conformational changes causing disengagement of the PHR domain and CCE that is accompanied by the formation of functionally active oligomers of cryptochrome molecules. Photooligomerization is considered as a key process necessary for cryptochrome signaling activity.