Cooperative predators, army ants in unison can attack stoutly defended social insect colonies much larger than themselves. Yet from folktales to fieldnotes, the image of army ants has too often magnified their aggression and ignored their magnificent capacity for social cooperation. A veteran of thirty years' research on ants the world over, the author offers the first comprehensive account of their behavioural ecology, ontogeny, morphology and evolution, and demonstrates the important role they have to play in both tropical ecosystems in general and maintaining species diversity in particular.