They have a stocky body, a slightly flattened head and are round nosed. Eyes have a red iris with vertically slit oval pupils. Colouration varies regionally and also between the sexes, males are more varied in colour than females. Their backs can be almost any colour from pale grey to a dark olive green with darker bands which provide a zig zag pattern, underneath they are a greyish blue with intense dark marks. They can also be entirely black; these are normally to be found in high regions around the tree line.

They live in a variety of habitats that include: chalky downs, rocky hillsides, moors, sandy heaths, meadows, rough commons, edges of woods, sunny glades, clearings, scrub-land and they can also be found in salt marshland but are not found in the warmer parts of France. It is likely that their range is decreasing with climate change.  The common adder, like most other vipers, is Ovoviviparous . Females breed once every two or three years giving birth  in late summer to early autumn producing between 3 and 20 young.

Prey, which is sought at dawn and dusk as a general rule,  consists of small mammals, birds, amphibians and lizards. Unlike other vipers it is a good swimmer and often takes to the water.

Hibernation takes place from October until March / April.

Adders are not usually aggressive, tending to be rather timid and biting only when cornered or alarmed. People are generally bitten only after stepping on them or attempting to pick them up. They will usually disappear into the undergrowth at any hint of any danger but can often return to the same favoured spot.

All French snakes are fully protected species