Being predatory lizards, goannas are often quite large, or at least bulky, with sharp teeth and claws. The largest is the perentie (V. giganteus), which can grow over 2.5 m (8.2 ft) in length. Not all goannas are gargantuan. Pygmy goannas may be smaller than a man's arm. The smallest of these, the short-tailed monitor (Varanus brevicuda) reaches only 20 cm in length. They survive on smaller prey, such as insects and mice. Goannas combine predatory and scavenging behaviours. A goanna will prey on any animal it can catch and is small enough to eat whole. Goannas have been blamed for the death of sheep by farmers, though most likely erroneously, as goannas are also eaters of carrion and are attracted to rotting meat.
Most goannas are dark-coloured, with greys, browns, blacks and greens featuring prominently; however, white is also common. Many desert-dwelling species also feature yellow-red tones. Camouflage ranges from bands and stripes to splotches, speckles, and circles, and can change as the creature matures, with juveniles sometimes being brighter than adults. Like most lizards, goannas lay eggs. Most lay eggs in a nest or burrow, but some species lay their eggs inside termite mounds. This offers protection and incubation; additionally, the termites may provide a meal for the young as they hatch. Unlike some other species of lizards, goannas do not have the ability to regrow limbs or tails.
Goannas are found throughout most of Australia, except for Tasmania, and manage to persist in a variety of environments. Most species are known to climb trees or outcrops; several primarily arboreal species are known. The lace monitor (V. varius) is probably the best-known among these, but is not the most common. The lace monitor is the second-largest of all goannas, reaching lengths of up to 2 m. Other more common tree goannas, such as the Timor tree monitor (V. timorensis) and mournful tree monitor (V. tristis,) do not grow to quite such lengths, averaging a maximum of 61 cm, nose-to-tail.
Other goannas are adapted to swampy coastal environments, such as the mangrove goanna (V. semiremex). Further still, the Mertens' water monitor (water goanna – V. mertensi), found in lagoons and rivers across northern Australia, is streamlined for swimming, using its tail as a paddle. Most other goannas are good swimmers, but tend not to voluntarily venture into the water.
The diets of goannas vary greatly depending on the species and the habitat. Prey can include all manner of small animals: insects, smaller lizards, snakes, mammals, birds, and eggs.
Meals are often eaten whole, thus the size of their meals may depend on the size of the animals. Many of the small species feed mostly on insects, with some being small lizard experts. Many of the medium to large species will feed on whatever prey items they can catch. This includes eggs, fish (V. mertensi), birds, snails, smaller lizards, snakes, marsupials, and other small mammals, such as rodents. The giant perentie has been observed killing a young kangaroo, and then biting out chunks of flesh like a dog.
All species are carrion eaters, so will feed on the carcasses of dead animals, including livestock and other large creatures. The smell of rotting meat will attract these lizards.