This chapter provides an introduction to syntactic structure and how it is represented by tree diagrams. It begins (2.1) with an account of the Merge operation by which words can be combined together to form phrases, and phrases combined together to form clauses. It then goes on to examine the nature of heads, complements and specifiers (2.2), and the nature of projections (2.3). Next (2.4), it outlines eight ways of testing syntactic structure, before turning to look at how hierarchical structure (and the relation c-command) plays a central role in the description of phenomena like the licensing of polarity items (2.5) and the binding of anaphors (2.6). It also briefly touches on the possibility of developing a theory of bare phrase structure, and of treating syntactic structures as inherently unordered, with linearisation taking place in the phonological component. The chapter concludes with a summary (2.7), and a set of bibliographical notes (2.8). Accompanying free-to-download materials include a Workbook for students, and an Answerbook and set of PowerPoints for teachers: each of these contain a separate set of materials for each core section of the chapter.