This chapter reviews the history of headache classification as it evolved from expert opinion to evidence-based diagnostic criteria. The structure and use of the International Classification of Headache Disorders are discussed, including for migraine. Particular focus is placed on the emergence of chronic migraine as a defined diagnosis, and how multiple research studies shaped the development of its current diagnostic criteria. The differential diagnosis for chronic daily headache is outlined, with emphasis on distinguishing chronic migraine from other primary headache disorders and secondary headaches. Risk factors for and comorbidities of chronic migraine are discussed. The chapter concludes with an overview of our current understanding of the pathophysiology of migraine chronification.