This paper discusses the role of the self-access centre (SAC) in tertiary language learning and teaching, a role which has developed out of the changes that have occurred within the disciplines of Applied Linguistics and Education as well as from wider changes in technology and society itself. As the focus in language learning has moved, over the past thirty years, from the teacher to the learner, self-access language learning has emerged as a complement to the more traditional face-to-face learning model, with SACs now operating in many parts of the world. One aim of the study was to develop a theory of how a SAC operates, in other words to answer the question "What is a self-access centre?" Central to this is the place that the SAC plays in the learning and teaching process. The study suggested that the SAC plays four main roles: bringing together language learning and independent learning, enabling the learner to improve both linguistic proficiency and independent learning skills, providing the necessary resources and providing learner support. The study also identified a number of constraints that can hinder SACs from playing a fully effective and efficient role in the learning and teaching process.