Lexis is central to language acquisition, as knowing a sufficient number of words is crucial for effective communication. For instance, 98 per cent word familiarity is necessary to comprehend written texts. Fewer words may suffice for speech comprehension, though variation among learners is significant. This chapter examines how vocabulary influences foreign language acquisition, reviewing key findings and recent developments in the field. First, achieving proficiency requires mastering at least 2,000-word families in English. Second, this threshold varies by task and learning environment. Third, corpus studies show that there are some words that cover a large proportion of all words occurring in spoken and written texts and that they tend to cluster together in various categories of formulaic language, which may make up to a half of discourse. Fourth, deliberate and incidental learning are complementary, though explicit instruction accelerates vocabulary acquisition in instructional settings. Finally, this chapter considers how educators can integrate classroom teaching with extramural learning. Evidence is continuing to emerge that extramural activities such as watching TV, listening to songs, gaming and engaging in social media may be promising avenues for learning words . However, it is unclear exactly how much vocabulary is acquired in extramural settings, and there is a need for future research to focus more on the systematic investigation of extramural learning environments, across a range of languages and for different age-groups.