Emus are large birds. They have small vestigial wings and a long neck and legs. Their ability to run at high speeds is due to their highly specialized pelvic limb musculature. Their feet have only three toes and a similarly reduced number of bones and associated foot muscles; they are the only birds with gastrocnemius muscles in the back of the lower legs. The pelvic limb muscles of emus have a similar contribution to total body mass as the flight muscles of flying birds.

Emus have brown to gray-brown plumage of shaggy appearance; the shafts and the tips of the feathers are black. Solar radiation is absorbed by the tips, and the loose-packed inner plumage insulates the skin. The resultant heat is prevented from flowing to the skin by the insulation provided by the coat, allowing the bird to be active during the heat of the day. A unique feature of the emu feather is its double rachis emerging from a single shaft. The sexes are similar in appearance.

On very hot days, emus pant to maintain their body temperature. Their lungs work as evaporative coolers, and unlike some other species, the resulting low levels of carbon dioxide in the blood do not appear to cause alkalosis. For normal breathing in cooler weather, they have large, multi-folded nasal passages. Cool air warms as it passes through into the lungs, extracting heat from the nasal region. On exhalation, the Emu\'s cold nasal turbinates condense moisture back out of the air and absorb it for reuse.

Their calls consist of loud booming, drumming, and grunting sounds that can be heard up to two kilometers away. The booming sound is created in an inflatable neck sac.