The aim of the paper is to review scholarly attempts to define or explain vocational education and training and to develop a theoretical model to analyse national definitions or conceptions of VET [vocational education and training] and how they have changed over time. VET takes many forms and is, perhaps, the least unitary of education sectors. Based on a literature review of previous attempts to characterise VET, the paper suggests using a multi-perspective framework which combines: (a) an epistemological and pedagogical perspective; (b) a system and institutional perspective; and (c) a socioeconomic and labour market perspective to analyse VET. These perspectives can help to identify appropriate learning approaches, institutional solutions and forms of cooperation to work towards. In Volume 2 of this series, the approach is empirically tested and the different understandings of VET in 30 European countries are illustrated. [This research paper is the first in a series produced as part of the Cedefop project, "The changing nature and role of VET (2016-18)." The research was carried out by a consortium led by 3s Unternehmensberatung GmbH. For Volume 2, see ED586252. For Volume 3, see ED586256.]