The International Classification of Headache Disorders, second edition (ICHD-II) was the result of 4 years' work by a large number of headache experts from different parts of the world. This article summarizes the main new features of ICHD-II, compared with the original International Headache Society classification: better definition of migraine with aura, inclusion of chronic migraine, inclusion of a number of new primary headaches (SUNCT, hypnic headache, benign thunderclap headache, new daily-persistent headache, hemicrania continua), better definition of the secondary headaches, introduction of medication-overuse headache and of headache attributed to psychiatric disorder. An appendix defines a number of entities for research purposes. The new classification has already been translated into many of the world's major languages and many more are in the pipeline. It is enormously important that headache experts support and encourage the use of the new classification in order to develop a common knowledge base, and that they research ways of further improving it.