An heterocyclic compound is any organic compound in which the atoms are linked to form one or more rings, at least one of which includes one or more atoms other than carbon – commonly nitrogen, oxygen, or sulfur. Many such compounds are of great biochemical and industrial importance. As with alicyclic compounds, saturated heterocyclic compounds resemble their aliphatic analogues except for the effect of strain in small rings. There is also a large class of heterocyclic aromatic compounds, highly distinctive in their properties.