The slow worm (Anguis fragilis) is a reptile native to western Eurasia. It is also called a deaf adder, a slowworm, a blindworm, or regionally, a long-cripple and hazelworm. These legless lizards are also sometimes called common slowworms. The "blind" in blindworm refers to the lizard's small eyes, similar to a blindsnake (although the slowworm's eyes are functional).

Slow worms are semifossorial (burrowing) lizards, spending much of their time hiding underneath objects. The skin of slow worms is smooth with scales that do not overlap one another. Like many other lizards, they autotomize, meaning that they have the ability to shed their tails to escape predators. While the tail regrows, it does not reach its original length. In the UK, they are common in gardens and allotments, and can be encouraged to enter and help remove pest insects by placing black plastic or providing places to shelter such as piles of logs, corrugated iron sheets or under tiles. On warm days, one or more slow worms can often be found underneath these heat collectors. One of the biggest causes of mortality in slow worms in suburban areas is the domestic cat, against which it has no defence.

Slow worms have been shown to be a species complex, consisting of 5 distinct but similar species.