Heterocyclic compounds are substances which contain a ring structure as found in benzene and the aromatic compounds, or aromatic hydrocarbons, but in which other atoms than carbon, such as sulfur, oxygen or nitrogen are found as part of the ring. Some examples are pyridine (C₅H₅N) and pyrimidine (C₄H₄N₂).

Note that compounds such as cyclopropane, an anaesthetic with explosive properties, and cyclohexane, a solvent, are not heterocyclic, they are merely cycloalkanes. The suffix '-cyclic' implies a ring structure, while 'hetero' refers to an atom other than carbon, as above.

In chemistry, a heterocyclic ring is a ring-like backbone whose atoms are not all of the same element.

That is, unlike the carbon-only ring like in benzene and napthalene, there are other elements in the backbone, such as in pyrimidine and purine.