Agreement is among the most widely-researched issues in theoretical linguistics. In this introduction, we critically review some of the key issues, focussing on typological approaches to agreement, the role of agreement in establishing and maintaining reference, and the emergence of agreement diachronically. We point to the interplay of semantic, pragmatic and syntactic factors in shaping the way agreement systems function, and emphasize the need for more usage-based research in understanding the impact of extra-syntactic factors. We also argue for greater attention to lesser-researched languages, particularly those where features other than person are central in agreement relations. Finally, we offer a short synopsis of the contributions to this issue.