This chapter delves into the multifaceted concept of vocabulary, exploring what constitutes a ‘good’ vocabulary beyond mere word count. It examines the depth of word knowledge, including form, meaning, usage, synonyms, antonyms, and collocations. The chapter also details how words change form through inflection and derivation. Furthermore, it provides an in-depth look at phrasal verbs, their structure, idiomatic meanings, and grammatical behavior (separability). Finally, the chapter introduces modal verbs, explaining their functions, grammatical characteristics, and specific uses, alongside an exploration of various phrases and idioms and their figurative meanings.