The Unforgivable Curses are some of the most powerful known Dark Arts spells. Used by the books' villains, such as Voldemort and the Death Eaters and in some cases the Ministry of Magic, their use inspires horror and great fear amongst others. The curses are so named because their use is - except by Ministry authorisation - forbidden and unforgivable in the Wizarding world when used on another human or probably any sentient being. Use of any of these spells on any human being can carry a whole life tariff in the magical wizard's prison of Azkaban. These curses are thus very rarely used openly. However, in Deathly Hallows, the Unforgivable Curses are used liberally by good characters, ranging from Professor McGonagall with the Imperius Curse, to Harry effectively using the Cruciatus Curse. He also uses the Imperius curse on a goblin and a suspicious Death Eater during their disguised attack upon Gringotts Bank. Since the spells are very powerful, their use requires a strong desire to bring about the effects, a directed will, and great skill.

It is noted that to perform the Unforgivable Curses, the caster must "mean it". This means that they need to want the effects a fair amount for the effects to last. In Order of the Phoenix, Harry attempts to use the Cruciatus Curse on Bellatrix, but he doesn't 'mean it', as Bellatrix later instructs him to, and the effects are temporary, causing her to only feel the pain for a moment.

The use of the Unforgivable Curses has existed since the time before the light and was authorized against Voldemort and his followers by Bartemius Crouch Sr, during the First Wizarding War. Although not stated, it is possible that this was limited to the Imperius Curse (to force a Death Eater to submit to questioning or surrender) and to the Killing Curse (as a last resort). Shortly after his resurrection, Voldemort names two Death Eaters "killed by Aurors".

The Unforgivable Curses:
Avada Kedavra (the Killing Curse)
Crucio (the Cruciatus Curse)
Imperio (the Imperius Curse)