In chemistry, azo compounds generally have a molecular formula of the form R-N=N-R', in which R and R' can be either aromatic or aliphatic. The N=N group is called an azo functional group. The name azo comes from azote, an old name of nitrogen that originates in French and is derived from the Greek a (not) + zoe (to live).

Aromatic azo compounds (R = R' = aromatic) are usually stable and have vivid colors such as red, orange, and yellow. Therefore, they are used as dyes, which are called azo dyes. Azobenzene is a typical aromatic azo compound. Their colour orginates from absorbance in the visable region of the spectrum due to the delocalization of electrons in the benzene and azo groups forming a conjugated system.

Aliphatic azo compounds (R and/or R' = aliphatic) are rather unstable. At an elevated temperature or by irradiation, two carbon-nitrogen (R-N) bonds are cleaved simultaneously with the loss of nitrogen gas to generate carbon-centered radicals. Owing to this process, some aliphatic azo compounds are utilized as radical initiators. Azobisisobutylonitrile (AIBN) is a typical one and is widely used in industrial processes and in laboratory experiments.

Aromatic azo compounds can be synthesized by using an azocoupling reaction, that is, an electrophilic substitution reaction on aromatic rings with diazonium salts. An oxidation reaction of hydrazine (R-NH-NH-R') also gives an azo compound.

Because of their instability, especially for aliphatic ones, care should be taken with the handling of azo compounds or an explosion may occur.