Adders mate in the late spring to early summer. The males locate the females by following their scent and will travel for hundreds of metres a day to find a mate. If a rival male appears on the scene, the first adder to arrive will fight it. The males raise the upper part of their body into the air and carry out an 'adder dance', moving around each other in a threatening display and trying to push each other to the ground. There is no record of adders biting each other during these battles. The defeated male leaves in search of a new mate.
The male and female stay together for a few days after mating and the female gives birth to a litter 3 to 4 months later. there can be from 3 to 20 young in a litter and each young snake is born in a thin transparent sac from which they have to free themselves. Occasionally the young will manage to break out of the sac while still inside the pregnant snake.
Young adders shed their skin for the first time after a couple of days and stay near their mother at first. They are born ready to bite and inject venom and start hunting right away, eating smaller prey such as worms and spiders until they are big enough to hunt larger creatures.