The study of sentence structure, or syntax, overlaps in crucial ways with the study of word structure, or morphology, or the study of morphosyntax. This chapter describes at what linguists study when they study syntax, and along the way we will see how the study of syntax often overlaps with the study of morphology and other components of grammar. In addition to studying how words fall into different syntactic categories, linguists also study phrase structure, or how words are grouped together into larger syntactic units. The chapter provides phrase structure rules for English sentence structure that capture the basic structure of each phrase. Phrase structure trees allow to demonstrate that sentences are not simply strings of words in linear order; they are organized hierarchically, into units within other units. This tree allows us to illustrate yet another interesting property of syntactic structure, namely recursion, the property of phrase structure rules that enables the generation of phrases of infinite length.