Echidna species can be distinguished by their spines and by the shape and length of the beak. The short-beaked echidna (Tachyglossus aculeatus) has a straight, forward-pointing beak and a heavy coat of spines. It is fairly common in suitable habitats throughout Australia; it is also found in New Guinea, although little is known to science about its range and habits there. Much larger is the long-beaked echidna of New Guinea (Zaglossus bruijnii), which has a downward-pointing beak and smaller, less-numerous spines dispersed through its brown fur. Long-beaked echidnas live at a wide range of elevations, generally in forested areas and only where human populations are low. They are endangered because of hunting (echidnas are edible) and loss of habitat. A second species of long-beaked echidna (Z. attenboroughi) was first described scientifically in 1999. It is very poorly known but is distinguished from Z. bruijnii by its smaller size and by a shorter, straighter beak.

The short-beaked echidna is probably Australia's most widely distributed native mammal, but it is common only where hollow logs, underbrush, and caves allow it to find shelter and ample food in the form of ants, termites, and other invertebrates. It catches prey whole with its long, sticky tongue, but it may break larger, soft-bodied victims into smaller pieces with its beak. It can open its tiny mouth only wide enough to allow its wormlike tongue to protrude.

The short-beaked echidna's head-and-body length, including the rudimentary tail, is usually 30–45 cm (12–18 inches). Its body is covered with a combination of fur and spines (modified hairs). Echidnas from colder regions such as Tasmania have long fur that partially obscures the spines, whereas echidnas of arid zones can appear to be covered in spines to the exclusion of fur. Beneath the coat of spines is a well-developed subcutaneous muscle layer, which in part accounts for the animal's surprising strength. This muscle layer allows the echidna to alter the contours of its stout body and thereby wedge itself into cracks and between tree roots. Echidnas are also able to dig themselves quickly into the ground when disturbed. In doing so, they appear to sink straight down into the soil, and, once dug in, they are well camouflaged by their spines. This combination of spines, strength, and strategy makes the short-beaked echidna difficult prey, and in fact it does enjoy a fairly predator-free existence—although dingos (dingo) and nonnative foxes, as well as automobiles, are occasional hazards. High temperature is another hazard faced by short-beaked echidnas. They have few sweat glands and cannot pant to shed excess heat; thus, echidnas may die of heat stress if cool shelter is not found. If the temperature drops too low, torpor or hibernation results.

The long-beaked echidna (Zaglossus bruijnii) is found only on the island of New Guinea and is usually described as being about 60 cm (24 inches) in length, although one individual was recorded at 100 cm. Like the short-beaked echidna, it is highly variable in its fur and spine cover. The number of claws on the front digits also varies. Z. bruijnii may in fact represent more than a single species and can unequivocally be divided into subspecies on the basis of these characteristics. Generally, the long-beaked echidna's spines are much shorter and less numerous than those of the short-beaked echidna, and the fur ranges from medium to dark brown. The beak is similarly used to probe leaf litter of the forest floor for food. The tongue, however, is shorter than that of the short-beaked echidna and is covered with backward-pointing barbs used to hook earthworms.

The recently described Z. attenboroughi is about the size of a short-beaked echidna and has a fairly straight beak, although in other respects it resembles Z. bruijnii. There is too little known about it to describe its form and habits in any detail.

Echidnas appear to congregate only during the breeding season, when a female may be followed by a train of suitors. After a gestation period of about 23 days, the female usually lays a single leathery egg into a temporary pouch formed by abdominal muscles and subcutaneous mammary tissue. The egg is incubated for another 10 days before the tiny offspring hatches with the aid of an egg tooth and fleshy bulb (caruncle)—structural holdovers from the creature's reptilian ancestry. The young echidna is protected in a special nursery burrow, where it sucks milk from special mammary hairs (teats and nipples are absent). When the young echidna is fully covered by spines and fur and is capable of feeding, it leaves the burrow for a solitary life. Echidnas are very long-lived; one echidna was reliably recorded at 45 years of age in the wild, and one captive individual was well over 50 years old at the time of its death.

Echidnas constitute the family Tachyglossidae, and their only living relative is the platypus. Together these animals constitute the mammalian order Monotremata (monotreme). Echidnas probably evolved from some unknown monotreme ancestor during the first half of the Tertiary Period (65 to 1.8 million years ago). Echidnas' lack of teeth has hampered study of their evolutionary history, because teeth fossilize well and often help to determine relationships between mammals. The oldest known fossil echidna was recovered from an eastern Australian cave deposit from about 17 million years ago (about the middle of the Miocene Epoch). Although the material is fragmentary, it suggests that basic echidna characteristics, such as the birdlike, toothless skull and robust skeleton specialized for digging, had evolved by this time. Echidnas appear to have once been widespread and diverse, and one especially large form measured more than 1 metre (3.3 feet) in length. Most fossil echidnas (genus Megalibgwilia) of recent epochs represent a type intermediate between today's short- and long-beaked families.