This chapter provides the syntactic background and deals with syntax, features, and agreement in order to spell out the hypothesis of expressive syntax more precisely. It focuses on the notion of syntactic features and the operation of agreement, which will be the syntactic operation that is most crucial for the analyses in the case studies. Starting with the original conception, the chapter presents more recent approaches to agreement, which drop the biconditional between (un)interpretability and (un)valuedness, before discussing the direction of agreement and siding with the view that agreement looks upwards. Moreover, it is assumed that only a complementizer phrase is a boundary for agreement, while a determiner phraseis not. The chapter concludes with a brief discussion of some approaches that attempt to represent certain aspects of the context in syntax and investigate if they can be put to use for the following case studies.