A hedgehog is any of the spiny mammals of the subfamily Erinaceinae, which is in order Erinaceomorpha. There are seventeen species of hedgehog in fivegenera, found through parts of Europe, Asia, Africa and New Zealand (by introduction).

There are no hedgehogs native to Australia, and no living species native to the Americas. Hedgehogs share distant ancestry with shrews (family Soricidae), with gymnures possibly being the intermediate link, and have changed little over the last 15 million years.  Like many of the first mammals they have adapted to a nocturnal, insectivorous way of life. Hedgehogs' spiny protection resembles that of the unrelated rodent porcupines and monotreme echidnas.

Hedgehogs are easily recognized by their spines, which are hollow hairs made stiff with keratin. Their spines are not poisonous or barbed and, unlike the quills of a porcupine, cannot easily be removed from the hedgehog. However, spines normally come out when a hedgehog sheds baby spines and replaces them with adult spines. This is called "quilling". When under extreme stress or during sickness, a hedgehog can also lose spines.