A cockatoo is any of the 21 species belonging to the bird family Cacatuidae, the only family in the superfamily Cacatuoidea. Along with the Psittacoidea (true parrots) and the Strigopoidea (large New Zealand parrots), they make up the order Psittaciformes (parrots). The family has a mainly Australasian distribution, ranging from the Philippines and the eastern Indonesian islands of Wallacea to New Guinea, the Solomon Islands and Australia. The name cockatoo originated from the Indonesian name for these birds, kaka(k)tua (either from kaka "parrot" + tuwah or "older sister" from kakak "sister" + tua "old").

Cockatoos are recognisable by the showy crests and curved bills. Their plumage is generally less colourful than that of other parrots, being mainly white, grey or black and often with coloured features in the crest, cheeks or tail. On average they are larger than other parrots; however, the Cockatiel, the smallest cockatoo species, is a small bird. The phylogenetic position of the Cockatiel remains unresolved, other than that it is one of the earliest offshoots of the cockatoo lineage. The remaining species are in two main clades. The five large black coloured cockatoos of the genus Calyptorhynchus form one branch. The second and largest branch is formed by the genus Cacatua, comprising 11 species of white-plumaged cockatoos and four monotypic genera that branched off earlier; namely the pink and white Major Mitchell's Cockatoo, the pink and grey Galah, the mainly grey Gang-gang Cockatoo and the large black-plumaged Palm Cockatoo.