Complex molecules, consisting of many atoms chemically bonded together, may have atoms arranged linearly or in rings.Heterocyclic and homocyclic compounds are examples of molecules in which the constituent atoms form rings. The cyclic portion of the name indicates their ring nature. The hetero- (from Greek meaning different) or homo- (from Greek meaning same) prefix refers to the composition of atoms in the molecule.

Homocyclic compounds are molecules that are, or contain, ring structures that consist only of carbon atoms within the ring. An example is benzene. Benzene is a homocyclic compound of six carbon atoms bonded together in a hexagonal ring, with one hydrogen atom bonded to each of the six carbons. Benzene is a highly toxic and volatile compound sometimes used in cleaning solutions. Phenol is another common homocyclic compound. Phenol, sometimes used as an antiseptic, is a benzene ring with a hydroxide group substituted for one hydrogen in the ring. Cyclohexane is another homocyclic molecule. In contrast, heterocyclic compounds, or groups, are rings containing at least one non-carbon atom in the ring. An example is heterocyclic amines which are six-membered rings of five carbons and one nitrogen atom. Heterocyclic compounds have an immense range of properties and uses and even constitute their own branch of chemistry (heterocyclic chemistry). A small set of examples in which heterocyclic compounds are used includes dyes, photochromes of film, pesticides, antiviral medications, and food additives. Many biologically important molecules are heterocyclic.DNA nucleotide bases, for example, are heterocyclic molecules. Also, the development of heterocyclic antidepressants, namely Prozac, have contributed greatly to the treatment of disease.