This chapter discusses a fundamental assumption of Sign-Based Construction Grammar, the framework adopted in this book: language is an infinite set of signs, including lexical and phrasal signs. Lexical entries license lexemes, and constructions license constructs – phrases that consist of a mother plus one or more (phrasal or lexical) daughter nodes. Lexemes belong to syntactic categories like noun, verb, and preposition. We diagnose these categories based on combinatory requirements of the words in question. But we could not properly analyze English clauses and sentences if we viewed them as simply strings of syntactic categories. Instead, sentences in English have a kind of hierarchical structure called constituent structure, as indicated by phenomena like subject-verb agreement. A constituent is a series of words that behaves like an indivisible unit for certain syntactic purposes, e.g., serving as the ‘clefted’ constituent in the it-cleft construction. Among the constituents we have discussed are NP (typically a determiner followed by a nominal expression) and VP (a verb optionally followed by a NP, AP or PP).