The field of English for specific purposes (ESP), which addresses the communicative needs and practices of particular professional or occupational groups, has developed rapidly in the past forty years to become a major force in English language teaching and research. ESP draws its strength from an eclectic theoretical foundation and a commitment to research-based language education which seeks to reveal the constraints of social contexts on language use and the ways learners can gain control over these. In this chapter, I will briefly point to some of the major ideas and practices that currently influence ESP, focusing on needs analysis, ethnography, critical approaches, contrastive rhetoric, social constructionism, and discourse analysis. I then go on to look briefly at some of the effects ESP has had on language teaching and research, arguing that it has encouraged teachers to highlight communication rather than language, to adopt a research orientation to their work, to employ collaborative pedagogies, to be aware of discourse variation, and to consider the wider political implications of their role. Together these features of ESP practice emphazise a situated view of literacy and underline the applied nature of the field.