This document discusses the nomenclature of heterocyclic compounds. It begins by defining heterocyclic compounds as carbocyclic compounds where one or more carbon atoms are replaced by a heteroatom such as nitrogen, oxygen, or sulfur. The International Union of Pure and Applied Chemistry has worked to systematize the nomenclature of these compounds. Single three to ten-membered rings are named by combining prefixes with the name of the parent ring structure. Numbering and naming becomes more complicated for fused ring systems, though many are known by common names like indole or isoquinoline. Spiro heterocycles contain two rings fused at a common point and are named based on the number of spiro atoms and heteroatoms