In order to effectively study and manage headache disorders, diagnosis is essential. In both research and clinical arenas, separating secondary causes from primary headache disorders is a crucial first step, followed by further specificity within these broader categories. Historical approaches to classifying headache disorders culminated in the International Classification of Headache Disorders (ICHD), completed and published in 1988. This was revised as the International Classification of Headache Disorders, 2nd Edition (ICHD II) in 2004. The International Headache Society's Subcommittee on Classification began work on the 3rd edition in 2010, and has just published this online and in the journal Cephalalgia. The diagnostic criteria for more than 200 causes of headaches are based upon evidence when available, and fortunately, recent research in the field of headache medicine has produced data applicable to the refinement of classification of a number of primary and secondary headache disorders. Some areas, however, await further study, making classification more challenging. This article will attempt to provide an overview of the rationale behind the ICHD, a guide to its use, and a summary of important diagnostic features of the primary and secondary headaches, particularly where these have changed significantly in the ICHD III from ICHD II.