The adder, or northern viper, Vipera berus, is a member of the family Viperidae. Adults typically attain a length of around 55-60 cm, with females being slightly longer and more stoutly built than males. Lengths of up to 90 cm have been recorded (in females), although snakes over 65 cm are rare. Adders can be easily distinguished from other native snakes by their markings. There is a dark, almost always continuous zigzag (or pattern of joined diamond shapes) running dorsally from the neck to the tail. The flanks are marked with dark round blotches.

The dorsal zigzag often 'straightens out' on the neck, often ending in a forward-pointing spear shape. There is usually also a dark V, H, X or M shape on the rear of the head. In adult males, these dark markings are often black, contrasting strongly with the grey, whitish, silvery or buff ground colour. In females, the markings are usually deep reddish brown or dark brown, while the ground colour may range from sandy or yellowish-brown, to dark brown or sometimes orange-brown. Yound adders usually have similar markings and coloration to adult females. At birth, adders measure around 15 cm.

Melanism (a preponderance of dark pigment) seems to occur more regularly in adders than in other native snakes. Melanistic adders are black in appearance, but the usual zigzag markings are often just visible on careful inspection.

The adder has vertical elliptical pupils (unlike the round pupils of smooth and grass snakes), and the head is relatively broader, with a more pronounced neck. The scales are distinctly keeled (grass snakes also have keeled scales, smooth snakes do not) and the anal scale is undivided (in grass and smooth snakes it is divided). Sloughs (shed skins) are often easily identified by the zigzag which shows up against the light, by the head scales (adders have many more small scales on the head), or by the anal scale.

The adder is ectothermic and depends on external sources of heat to raise its body temperature in order to permit activity. Much of its behaviour is governed by the need to maintain a suitable body temperature; in studies, the mean selected body temperature has been reported as around 30°C, but adders may exceptionally be active in temperatures down to around 5°C during early emergence. Ectothermy restricts activity largely to the period between March and mid October, and to being almost entirely diurnal during this period. Adders generally bask in order to raise their body temperature, but they may also gain heat by contact with warm surfaces such as rocks or tarmac.

Males usually emerge from hibernation earlier than females, in mid February or early March. These dates can vary according to local weather conditions, however. Both sexes bask, or 'lie out' for a short period in spring during which time they do not feed; they then shed their skins and become more active (again, males do this several weeks before females) in preparation for mating. Adults of both sexes will slough once more in the summer. Mating usually occurs from mid April to May, and may be preceded by the male combat ritual in which rival males entwine the anterior halves of their bodies and attempt to push each other down to the ground. Males follow scent trails left by females, and copulation itself may last for two hours. During copulation, males pass a secretion which causes the muscles in the female's reproductive tract to contract, and this is thought to act as a "plug" to reduce the chance of other males fertilising her eggs should she copulate subsequently. The eggs are retained inside the female and live young are born from late August to September, usually close to the hibernation area. There are usually 6-10 (range: 3-20) per litter with females generally only giving birth every other year.

Feeding commences after mating. The diet is mostly small mammals and reptiles, particularly mice, voles and common lizards. The proportion of these taken seems to vary according to age, with young taking more lizards and adults concentrating on small mammals. They also take fledgling birds, and occasionally frogs. Adders can swim well, but this is generally a much rarer component of their behaviour than is the case for grass snakes, which habitually prey on amphibians. The annual intake of an adult adder has been estimated as the equivalent of nine field voles per year.

Adders may grow around 10 cm per year in the first 3-4 years, at which time they reach sexual maturity; males generally mature a year earlier than females. The longevity of wild adders is largely unknown, but it is unlikely that many individuals reach 15 years. Mortality is highest during the first year of life. Predators include many raptors, corvids, pheasants, foxes and hedgehogs.

The use of distinct summer and winter ranges has been reported and these can be separated by distances of over a mile. One study in Dorset showed movement between wet bog areas, used in the summer, to a dry, wooded area in the autumn to spring period. In Essex similar movements between dry wooded areas (over winter) and grassy meadows in summer were observed.

The migration between summer and winter habitat no doubt varies between regions; indeed this movement between well drained hibernating areas and damper summer areas might not be universal, especially where these habitats are close or overlapping. However it is likely that adders remain in the vicinity of the hibernation site (or "hibernaculum") in early spring and return there in autumn.

Hibernation sites are often associated with banks or tumuli, often south facing. Generally these are in well-drained soil, often with woodland or scrub, and the vegetation can be quite dense. Earth banks are frequently used and holes in the ground or fissures, such as those associated with tree roots or mammal burrows, are occupied. Old stone walls or piles of rubble may also be used. It appears the main criteria are that the site should be easily penetrable, avoid becoming waterlogged, provide a frost-free hibernating den and be in a relatively sunny position. Adders may hibernate communally, sometimes along with other reptile species.