The debate which surrounds the role of English as a lingua franca has been lively and extensive over the last twenty years or so. Numerous conferences and specific publications in the field testify to the interest and importance that the question of the development of a possible international functional language has stimulated. The present article aims to critically reassess the claims made for ELF and, at the same time, highlight the profound significance of the issues the debate has raised. Discussion of ELF has touched upon the perception of linguistic diversity, (in)accuracy and variation, intelligibility and the potential functions of an international language. Other topics of profound importance that are part of the debate relate to the role of any language in creating and maintaining identity, enabling social mobility and empowering its users. Some criticisms of ELF proposals have been easily rebutted, but this paper aims to underline the serious linguistic and socio-linguistic aspects of the debate in order to emphasize the need for a theoretical underpinning to our understanding of how language can operate in the global environment. Clarification of what we can and cannot know about language change and behaviour will also contribute to ideas as to future research in ELF but will also bring us to the conclusion that predictions of the future development of ELF are of little purpose beyond the curiosity value they may possess.