Indole is a fused heterocyclic ring with the empirical formula C8H7N. Indole was first discovered from indigo dye (thus its name), which is two modified indole molecules connected together. Indole shares many of the same basic physical and chemical properties with benzene and other aromatic rings, although the presence of a nitrogen atom in one of its rings make it slightly more complicated.

Indole is most important biologically because it is the R-Group of tryptophan. This makes it the largest and most complicated side chain of any of the amino acids. The indole ring is the only fused ring side chains amongst the amino acids. Because tryptophan is ubiquitous amongst living things, indole rings are used as building blocks in a number of chemicals, many with quite different properties. As mentioned, indigo dye is based on an indole ring structure, and there are many other variations on indole-ring based pigments in the natural world, including melanin. A number of artificial and natural perfumes (such as jasmine) contain indole rings as well. And since tryptophan is a precursor to serotonin, many drugs that work on serotonin are based around an indole ring, including LSD, DMT and psilocybin.

The diversity of structures made out of a basic building block such as the indole ring is one of the reasons why biochemistry is such a rich and sometimes daunting field. There is no particular reason that the dye that makes blue jeans blue should be based around the same chemical structure as a potent, complicated drug such as psilocybin, and yet this is the case.