Introduction English for academic purposes (EAP) can be conceived as a field of academic knowledge (what constitutes academic English) and a field of practice (the teaching and learning of academic English). Both fields are addressed in this chapter from the perspective of systemic functional linguistics (SFL), a social semiotic theory of language. This is primarily a linguistic take on EAP, although studies that explore other semiotic systems such as body language and image are also noted. In contributions to practice, the main focus is on tertiary contexts and transitions towards that sphere. (Humphrey, this volume, takes a school focus on EAP.) In discussing contributions from SFL, a social semiotic toolkit for undertaking research and for informing teaching of EAP emerges, as do insights into key concerns for the field including the significance of disciplinary specialization, issues of stance and identity, and the management of diverse and changing technologies in pedagogic interactions.