Abstract: Army ants are dominant social hunters of invertebrates and thereby play an integral role in tropical ecosystems. They are defined by a suite of evolutionarily interrelated physiological, behavioural and morphological traits, the army ant adaptive syndrome: they are obligate group predators, they frequently relocate their nests, and their permanently wingless queens found new colonies accompanied by workers. If this functional definition is applied rather than a taxonomic one, army ants have evolved repeatedly in distantly related groups of ants. In addition, army ants typically have extremely male-biased numerical sex-ratios, and the queens of the studied species are inseminated by many males. The aim of this review is to provide a synthesis of the most recent work on army ant biology, to outline an evolutionary scenario that connects the different aspects of army ant life-history, and to give some directions for future research.