Flavins are a family of yellow-colored compounds with the basic structure of 7,8-dimethyl-10-alkylisoalloxazine. Riboflavin, commonly known as vitamin B2, is an essential component of living organisms and is the precursor of all biologically important flavins. In this chapter, the redox properties of flavins are described, with special emphasis in their ability to participate in both one-electron and two-electron transfer processes; hence, flavins are indispensable mediators between two-electron and one-electron processes in biological systems. The photophysical and photochemical properties of flavins are also discussed. All oxidized flavins exhibit strong absorption in the ultraviolet and visible regions and an intense yellow-green fluorescence (in their neutral oxidized form). Flavins are thermostable compounds; however, they are photosensitive. In the absence of an external reductant, the isoalloxazine ring system undergoes intramolecular photoreduction. Some flavins are efficient photosensitizers; they can induce photomodifications of compounds that are not directly modified by visible light.