Adders live in open heath or moorland habitats, and many of these have been lost under forestry plantations, farmland, development or encroaching scrub.
Females are nearly always a red-brown earthy colour with the males being slightly smaller and a much lighter colour, most often a striking slate grey.
Both sexes have defined black zigzags down their backs. The pattern resembles a shadow cast by a fern, which is why a lot of people, and predators, don't see them.
Adders are very sparse in mid-Wales, but they can be seen in reasonable numbers in the north and south of the country. They live in a variety of open or semi-open habitats, from heathland, young woodland, golf courses and even motorway verges.
Adders are our only venomous snake, but they rarely bite unless they are picked up or stepped on. Their venom is not usually life threatening, but can make you feel unwell and medical attention should be sought.
Snakes don't hibernate, they 'brumate' over winter. In other words, their body temperature is maintained at a cooler level than normal, but they are awake, and slowly move around to keep warm.
Try one of the walks from Derek's latest walking series on BBC One Wales.
Find out about the wildlife you can find on your doorstep.