The International Headache Society's (IHS) Classification of Headache Disorders, published in 1988, is largely responsible for stimulating the rapid scientific and therapeutic advances that have revolutionized the field of headache. By establishing consistent operational diagnostic criteria for primary and secondary headache disorders, the IHS Classification has facilitated epidemiological and genetic studies as well as the multinational clinical trials that provide the basis for our present treatment guidelines. Fifteen years after its original release, a revised 2nd edition has been unveiled. Modifications are small but significant. We hope to introduce clinicians to the salient changes in the 2nd edition by highlighting the newly included headache types, acknowledging the renamed headache types, and reviewing the modifications in diagnostic criteria for existing headache types. Physicians involved in the care of headache patients need to be aware of these changes and should continue to consult the IHS criteria to ensure accurate diagnosis, to continue to refine the diagnostic criteria, and to contribute to the body of knowledge necessary to make further advances in the classification as well as in the field of headache.