Cytochromes are, in general, membrane-bound hemoproteins that contain heme groups and carry out electron transport.

They are found either as monomeric proteins (e.g., cytochrome c) or as subunits of bigger enzymatic complexes that catalyze redox reactions.

Cytochromes were initially described in 1884 by MacMunn as respiratory pigments (myohematin or histohematin). In the 1920s, Keilin rediscovered these respiratory pigments and named them the cytochromes, or “cellular pigments”, and classified these heme proteins, on the basis of the position of their lowest energy absorption band in the reduced state, as cytochromes a (605 nm), b (~565 nm), and c (550 nm). The UV-visible spectroscopic signatures of hemes are still used to identify heme type from the reduced bis-pyridine-ligated state, i.e., the pyridine hemochrome method. Within each class, cytochrome a, b, or c, early cytochromes are numbered consecutively, e.g. cyt c, cyt c₁, and cyt c₂, with more recent examples designated by their reduced state R-band maximum, e.g. cyt c₅₅₉.