Isomerism, phenomena in which compounds share the same molecular formula (isomers) but are otherwise different, and isomerization, transformations between isomers, is a fundamental aspect of Chemistry. The two major forms of isomerism -constitutional isomerism (involving isomers that have different connectivity) and stereoisomerism (involving isomers with the same connectivity but different spatial arrangement) -have been treated as distinct and isolated domains of study, with separate theoretical frameworks. This division, however, has long limited a comprehensive understanding of isomerism. Herein, a unifying framework that applies common principles to both these types of isomerism is introduced, overcoming this artificial separation thus providing a unified view of molecular isomerism.