This article is an attempt to apply distinctive feature theory to the study of diphthongization. It shows that several types of phonological change—some of them traditionally called diphthongization, others not—involve identical processes of change, and differ only in terms of the distinctive features involved. It identifies the single principle of intra-segmental variation which underlies all such changes, and thus defines the place of diphthongization in a general typology of phonological change.