Teachers of English as a second or foreign language often state that they lack an understanding of how to teach vocabulary in a principled, evidence-based way sensitive to students' needs. Vocabulary teaching is typically unsystematic, not adequately supported by curricula and teaching materials, and shaped by beliefs based in opinion or myth. A large amount of research on L2 vocabulary learning and processing is now available, and most of this work is on English vocabulary. The present article synthesizes this body of knowledge to achieve the following: (a) establish how many words learners need to know for different purposes; (b) discuss the scientific evidence for commonly held beliefs about vocabulary teaching; (c) recommend sound, research-informed teaching practices; and (d) refer the audience to a range of freely available high-quality tools that can facilitate lexical instruction in English.