The homology (homolo) relation between molecules was introduced. This relation is a generalization of an old idea of series of homologous compounds. The homolo relation operates on a molecule by removing all the structural fragments that are identical with a certain selected fragment. As a result a multiset of fragments is produced. It was shown that the homolo relation is an equivalence relation in a set of molecules. Thus, by choosing various reference fragments, the molecules can be classified in many different ways. Using the language of homolo operation it is possible to redefine such ideas as constitutional and stereo isomers as well as a generator of a molecule and, for instance, factorization of a molecule onto fragments. </p
