Albus Dumbledore mentions in his notes on Beedle the Bard that Necromancy is "a branch of magic that has never worked". However, it is not known to what extent he was telling the truth with reference to this statement - he does not reveal all he knows in his notes. Whilst it is possible to create Inferi, these are mindless creatures with no soul or intelligence, despite possessing human corpses and hence are not truly "raised" persons. Therefore, Necromancy does not work in this respect and it is indeed known to be impossible to truly resurrect the dead. However, zombies are known to exist in the Harry Potter universe and may be the result of true necromancy (though, the extent of their humanity remains to be seen).