This chapter begins by providing a brief history of authenticity in language learning and outlines some of the reasons for its growing importance in the field including: (i) a greater focus on communicative competences in ELT; (ii) technological developments in tape/video recording which have spawned new fields of enquiry such as discourse or conversational analysis, providing deeper insights into real-time oral interaction; (iii) advances in information and communications technology (ICT) which have provided materials designers, language teachers and learners with unfettered access to authentic input from across the globe. It then attempts to tease apart, and make sense of, the tangled web of meanings around this multidimensional concept, describing seven definitions of authenticity which exist in the ELT literature and their implications for language teaching and teacher training. Finally, sample materials are provided to illustrate how some of the principles of authenticity highlighted in the chapter can be realized in a genuine classroom context. They show how multimodal authentic materials often provide a richer source of input than conventional textbooks, which allows learners to ‘notice’ different L2 features, depending on their own particular stage of interlanguage development or interests, and to develop a wider range of communicative competencies.