Blood purity is a racist concept in the fictional Harry Potter book series.

In the books, wizards live in voluntary segregation from the Muggle (non-magical) society. It's very common to view Muggles as disadvantaged, lacking the ability to perform magic; when taken to extremes, this becomes the belief that Muggles are inferior and that marriage between wizarding and Muggle people taints the magical race.

The general public - including the good characters - considers the notion obsolete and false, but some whom would benefit the most from it strongly disagree. There is some evidence that the idea of blood purity has enjoyed greater support in the past and is in decline as Muggle ancestry becomes more and more common. If magical aptitude is indeed partially dependent on one's heritage, then this part is overshadowed by variation from individual to individual.

The wizarding world is divisible into three main classes by purity.

Pure-Blood

Pure-blood wizards have no Muggles whatsoever in their genealogical pedigree. The few remaining families are all interconnected. Unsurprisingly, the strongest proponents of blood purity are pure-blood themselves and a few extremists have even campaigned for the reclassification of muggles as unintelligent beasts as opposed to beings.

By no means is pure-blood status any guarantee of a person's attitude. Attitudes do tend to run in families as children are brought up to support their parents' views, but even families that consider Muggle marriage disgraceful and immoral have their black sheep. Blood traitor is a degoratory term for a pure-blood who fails to maintain the proper prejudices.

Known Pure-Blood Families

The Black Family

• Sirius Black
• Regulus Black
• Sirius's Mother , name unknown
• Uncle Alphard
• Aunt Elladora
• Araminta Melliflua
• Phineas Nigellus
• Bellatrix Lestrange
• Andromeda Tonks (married Ted Tonks, a Muggle-born; daughter Nymphadora Tonks is half-blood)
• Narcissa Malfoy

The Malfoy Family

• Lucius Malfoy
• Narcissa Malfoy
• Draco Malfoy

The Weasley Family

• Arthur Weasley
• Molly Weasley
• Bill Weasley
• Charlie Weasley
• Percy Weasley
• Fred and George Weasley
• Ron Weasley
• Ginny Weasley

The Longbottom Family

• Neville Longbottom
• Neville's Grandmother
• Frank Longbottom
• Alice Longbottom

The Crouch Family

• Barty Crouch
• Barty Crouch Jr.
• Mrs. Crouch

The Lestrange Family

• Rodolphus Lestrange
• Rabastan Lestrange

The Death Eaters

Main article: Death Eaters

It's not known if all members are pure-blood, but as the "purification" of the wizarding race is a stated goal of their leader, Lord Voldemort, it seems rather likely. See the main article for the names of the group's members.

Squib

Squibs are the opposites of Muggle-borns and a rare anomaly in the classification system. A Squib is of wizard heritage but lacks magic, though they seem to be able to perceive the magical things that Muggles cannot (notably ghosts). Many (if not most) live an unfortunate life in the wizarding world.

It has been theorized that Squibs may hold a special affinity for cats due to the close relationships with cats that both known Squibs have, and that Squibs are the result of inbreeding. Both theories remain unproven.

Known Squibs

• Argus Filch
• Mrs Arabella Figg
• Neville Longbottom was falsely suspected for a time.

Half-Blood

Half-bloods have more Muggle ancestry than pure-bloods, but less than Muggle-borns. The exact amount that disqualifies a person from pure-blood status is unknown, but Muggle grandparents are enough and it's quite possible that even one ancestor suffices. Half-bloods are the most common kind as the magical folk would have dwindled to extinction without marrying Muggles. Blood purity fanatics regard half-bloods as an inferior kind of wizard, but still a wizard.

Known Half-Bloods

• Harry Potter: James Potter was pure-blood, Lily Evans Muggle-born.
• Tom Riddle (alias Lord Voldemort): Father, Tom Riddle Sr., was a Muggle, mother pure-blood.
• Nymphadora Tonks: Father, Ted Tonks, was Muggle-born, mother, Andromeda Black, pure-blood.
• Seamus Finnigan: Muggle father, pure-blood mother.
• Dean Thomas (Muggle mother, father a wizard wrongly presumed to be a Muggle.)

Muggle-born

Muggle-borns are the occasional witches and wizards born to Muggle parents. What causes them is unknown, likely due to the intentional obscurity of magic's technical details. Though their magical abilities do not seem to be significantly affected, blood purity fanatics regard them as little better than the sub-wizard Muggles and they are the most persecuted type of magical persons. Mudblood is a highly insulting expression for a muggle-born that more or less identifies the user as prejudiced.

Known Muggle-borns

• Hermione Granger
• Lily Potter
• The Creevey Brothers
• Justin Finch-Fletchley