This Element examines the foundational building blocks of English for Specific Purposes (ESP) teaching. The emergence of ESP teaching as a global movement has been driven by economic, social, and educational factors. Currently, examples of ESP teaching can be seen across a wide variety of learner groups and contexts. Underlying this variety, two core concepts unify the field – teaching addresses learners' work- or study-related language needs, and teaching targets specialized English. These mainstay concepts have come to assume a taken-for-granted status in the field, and recent discussion and analytical review of them has been limited. The Element scrutinizes the concepts, examines the ideas behind them, identifies potential issues in their application and attempts to forge new links. Teaching contexts vary but needs analysis and specialized English unify the field of English for Specific Purposes.