The hypothesis that pachycephalosaurid dinosaurs used their massive domed heads in agonistic head-to-head butting is reexamined. It is questionable whether such behavior was possible because of the small contact area of opposing heads. Instead, flank-butting is suggested based on analogy with extant African antelopes. Two types of flank-butting are recognized on the basis of dome structure. Type One is seen in Pachycephalosaurus, Prenocephale, and Stegoceras, all of which have tall, rounded frontoparietal domes. Such domes would maximize the mass of the head in flank-directed blows. Pain would be felt by the opponent without causing serious injury. Type Two is seen in Stygimoloch, in which squamosal horns are developed along the posterior margins of the dome. These horns seem to be an adaptation for causing maximum pain locally during flank-butting without causing serious injury. The horns probably were not sharp, but blunted.