This chapter provides an introduction to the distinction between lexical and grammatical words. It gives an overview of features that are characteristic of the two types of words. It discusses the differences between the lexical–grammatical distinction and related distinctions between content and function words, and open- and closed-class words. It goes through a number of definitions of the lexical–grammatical distinction. It also discusses to which extent word classes like nouns, verbs, adjectives, pronouns, and adpositions are homogeneous with respect to the distinction. Finally, it considers similarities between the lexical–grammatical distinction and word–class distinctions with special focus on the notions of structured complexity and division of labour, and in relation to holophrases.