Headache is extremely prevalent and experienced by almost everyone. A clinical test for the diagnosis of headache is still lacking, and a detailed clinical description is therefore essential for a precise diagnosis. The second edition of the International Headache Classification is presented, along with a translation into Danish. The major changes from the first edition are a detailed description of several subtypes of migraine, three subdivisions of tension-type headache according to frequency of headache and a major extension of the chapter concerning secondary headaches due to medication overuse. General knowledge of the major subforms of primary and secondary headaches is a very important tool for diagnosis and treatment.