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Following a short analysis of leading principles of the classification and diagnostic criteria of headaches of the International Headache Society (IHS) and a brief description of its nosography of primary headaches, some still debated issues are discussed. The IHS classification shows a good inter-observer reliability both at one- and two-digit levels, even though validation studies are lacking. On the basis of researches carried out in recent years on the evolution of cluster headache, on chronic daily headache, on a possible third diagnostic level of migraine without aura, and on the painful phase of migraine with aura, some proposals of revision of the IHS classification are suggested.