Leafcutter ants are several species of fungus-growing ants that share the behaviour of cutting leaves which they carry back to their nests to farm fungus. Next to humans, leafcutter ants form some of the largest and most complex animal societies on Earth. In a few years, the central mound of their underground nests can grow to more than 30 m (98 ft) across, with smaller radiating mounds extending out to a radius of 80 m (260 ft), taking up 30 to 600 m² (320 to 6,460 sq ft) and occupied by 3.55 million individuals.

Leafcutter ants are any of at least 55 species of leaf-chewing ants belonging to the three genera Atta, Acromyrmex, and Amoimyrmex, within the tribe Attini. These species of tropical, fungus-growing ants are all endemic to South and Central America, Mexico, and parts of the southern United States. Leafcutter ants can carry up to 50 times their body weight and cut and process fresh vegetation (leaves, flowers, and grasses) to serve as the nutritional substrate for their fungal cultivates. The leaf cutter ant species has a bite force of 800 mN, which is 2600 times their body weight, which allows them to cut leaves as well as defend the nest.

Acromyrmex and Atta ants have much in common anatomically; however, the two can be identified by their external differences. Atta ants have three pairs of spines and a smooth exoskeleton on the upper surface of the thorax, while Acromyrmex ants have four pairs and a rough exoskeleton. The exoskeleton itself is covered in a thin layer of mineral coating, composed of rhombohedral crystals that are generated by the ants. Amoimyrmex and Acromyrmex differ in that Amoimyrmex lacks tubercles on the first gastral segment, and recent phylogenetic evidence shows that Amoimyrmex diverged before the other two genera of leafcutter ants.

Winged females and males leave their respective nests en masse and engage in a nuptial flight known as the revoada (Portuguese) or vuelo nupcial (Spanish). Each female mates with multiple males to collect the 300 million sperm she needs to set up a colony.

Once on the ground, the female loses her wings and searches for a suitable underground lair in which to found her colony. The success rate of these young queens is very low, and only 2.5% will go on to establish a long-lived colony. To start her own fungus garden, the queen stores bits of the parental fungus garden mycelium in her infrabuccal pocket, which is located within her oral cavity. Colonies are generally founded by individual queens — haplometrosis. Because colonies with multiple queens over the lifespan of the colony have been found by a large number of investigators – by Weber (1937), Jonkman (1977), Huber (1907), Moser & Lewis (1981), Mariconi & Zamith (1963), Moser (1963), and Walter et al. (1938) — it is believable that some colonies have multiple foundresses — termed pleometrosis. Colony founding by pleometrosis has only been confirmed in Atta texana, by Vinson (1985).

In leafcutter colonies, ants are divided into castes, mostly of different sizes, that perform different functions. Acromyrmex and Atta exhibit a high degree of polymorphism, four castes being present in established colonies — minims, minors, mediae, and majors, also known as soldiers or dinergates. Atta ants are more polymorphic than Acromyrmex, with less difference in size from the smallest to largest types of Acromyrmex.^{[citation needed]}

Their societies are based on an ant–fungus mutualism. The only two other groups of insects to use fungus-based agriculture are ambrosia beetles and termites. Different species of ants use different species of fungus, but all of the fungi the ants use are members of the family Lepiotaceae. The ants actively cultivate their fungus, feeding it with freshly cut plant material and keeping it free from pests and molds. This mutualistic relationship is further augmented by another symbiotic partner, a bacterium that grows on the ants and secretes chemicals; essentially, the ants use portable antimicrobials. Leaf cutter ants are sensitive enough to adapt to the fungi's reaction to different plant material, apparently detecting chemical signals from the fungus: if a particular type of leaf is toxic to the fungus, the colony will no longer collect it. The fungus cultivated by the adults is used to feed the ant larvae, and the adult ants feed on leaf sap. The fungus needs the ants and the larvae need the fungus; mutualism is obligatory.

The fungi used by the higher attine ants no longer produce spores. These ants fully domesticated their fungal partner 15 million years ago, a process that took 30 million years to complete. Their fungi produce nutritious and swollen hyphal tips (gongylidia) that grow in bundles called staphylae, which have no other function than to feed the ants. Leucoagaricus gongylophorus is the most commonly documented fungi farmed by higher attine ant species.