Agreement in language occurs when grammatical information appears on a word which is not the source of that information. In simple examples like she runs, the form runs is singular, agreeing in number with she. This is information about the number of runners (just one), and it matches that expressed in its source she. Patterns of agreement vary dramatically cross-linguistically, with great diversity in expression and types of variation found. This clear introduction offers an insight into how agreement works, and how linguists have tried to account for it. Comparing examples from a range of languages, with radically different agreement systems, it demonstrates agreement at work in a variety of constructions. It shows how agreement is influenced by the conflicting effects of sentence structure and meaning, and highlights the oddities of agreement in English. Agreement will be essential reading for all those studying the structure and mechanisms of natural languages.