The purpose of this paper is to establish a coherent and uniform explanation of the problematic issue of the different Doric long-vowel systems and their development. To achieve this, it will analyse both the socalled second compensatory lengthening in Insular Doric and Elean and the monophthongization of the former hiatus vowels, as well as their impact on the phonological system, since both processes are crucial for the shift from a media system to a severior system. This phenomenon will be explained by means of the Theory of Neutralizations, whereby the ancient Doric system with seven degrees of aperture was simplified in several dialects due to a trend towards the elimination of the contrast between mid-closed vowels. In addition, some other complex aspects of Doric vocalism will be discussed, including the so-called Doric compensatory diphthongs, the Cyrenaean <E> spelling, the Elean /æː/ sound, and the central Cretan long vowels.