Rheas belong to the order Struthioniformes, or ratites. There are two genera within this family, Rhea and Pterocnemia, and most classification schemes recognize one species within each genus. They are found exclusively in South America, usually in open grasslands (campo).
Rheas are tall, long-legged birds, well adapted to a terrestrial lifestyle. They are excellent runners, achieving speeds of up to 60 km/h. Their wings lend them great maneuverability while running. These paleognath birds are characterized by large, fenestrated maxillo-palatines that do not articulate with the vomer, and short palatines that do articulate with the vomer. They have a tracheobronchial syrinx with one pair of intrinsic muscles and modified bronchial and tracheal rings. Their tarsi are scutellated, and like most other ratites, they have three toes and short middle phalanges. Their plumage is greyish-brown, and the sexes are not very dimorphic, though males develop a black ring on the neck when in breeding plumage. Rheas lack oil glands, and their feathers lack hooks, making them unusually soft. Unlike the similar-looking ostriches, the head, neck, and thighs of rheas are feathered. They do not have retrices (tail feathers), but do have a claw on each wing.