Some reflections on the nature of ELF. The focus in this article is what scholars of English as a Lingua Franca (ELF) have referred to as ‘unilateral idiomaticity’ (UI): the use by speakers and writers of idiomatic language which is not understood by the other participants in the interaction. The writer will attempt to identify actual occurrences of UI, drawing on both corpus evidence and an email survey of fifty L1 and L2 ELF-users and will also draw some implications for what we mean by ‘competence’ in lingua franca contexts.