The longstanding separation between the wizarding and Muggle worlds in the Harry Potter universe has led many wizards to advocate keeping the two apart. Wizards of pure Muggle parentage are viewed as untrustworthy, foolish, or, in extreme cases, racially inferior. The common practice of wizards marrying Muggles is viewed by such extremists as miscegenation, and they instead advocate maintaining a so-called "purity of blood". This was part of Lord Voldemort's ideology, and the Black family disowned anyone who married a half-blood or muggle. However, in Harry Potter and the Chamber of Secrets, Hagrid and Ron both state there is probably no such factor as "blood purity", with all wizards (given sufficient research) likely to find in their family history some marriages to Muggles.

Pure-blood

Pure-blood is the term applied to wizards and witches who have no Muggle blood, Muggle-borns, or half-bloods at all in their family tree. They are the rarest of the three blood statuses, with J.K. Rowling saying 10% of the Wizarding community is made up of pure-bloods. Although technically pure-bloods have no Muggle ancestors, the small wizarding population means that "true" pure-bloods are rare, with some just ignoring or disowning the few Muggles in their family. Identified pure-blood families include the Blacks, Crouches, Fudges, Gaunts (though that line died out before the beginning of Philosopher's Stone), Lestranges, Longbottoms, Malfoys, and Weasleys (but the most recent generation has had half-blood members). To maintain their blood purity, some supremacist families have been known to inbreed into their own families by marrying their cousins, resulting sometimes in mental instability and violent natures. The Gaunt family displayed both tendencies by the time of the sixth book.

Pure-blood supremacists believe blood purity is a measure of a wizard's magical ability – notwithstanding examples of skilled Muggle-born witches and wizards such as Hermione Granger and Lily Potter – and less skilled pure-bloods such as Neville Longbottom (whose skills developed in Deathly Hallows due to his heroism) – and Muggles to be low-life, having no magic in them. Supremacists apply the term "blood traitor" to pure-bloods who married and had children with non-pure-bloods.

The pure-blood wizards and witches in the Harry Potter books are almost all supremacists, while there are some of them who don't advocate ancestral superiority; the Weasleys, Potters, and Longbottoms are old pure-blood families, but no known members of these families are sympathetic to supremacist aims. The Black family, traditionally pure-blood supremacists, also seem to have produced one or two such "black sheep" in every generation, namely Sirius and Andromeda (Bellatrix and Narcissa's sister who married the Muggle-born Ted Tonks).

Several wizards question the notion of blood purity altogether. In his copy of The Tales of Beedle the Bard (later bequeathed to Hermione Granger), Dumbledore has made annotations that he thinks the much-vaunted blood purity does not exist and is only a fiction maintained by the deceptions of supremacist wizards.

Half-blood

Half-blood refers to those wizards and witches who have magical and Muggle ancestors in their family trees. Half-bloods are the most common wizard blood, far outnumbering pure-bloods and Muggle-borns. Rowling has stated that, of the Hogwarts annual intake, 50% are half-bloods. Pure-blood supremacists view half-bloods as inferior to them, although superior to Muggles and Muggle-borns. The Malfoy family, a family of wealthy Wizards who showed disdain to Muggles, attempted to maintain blood purity but found it acceptable to marry half-bloods if there was a dearth of marriageable pure-bloods.

Voldemort is a half-blood, and his most guarded secret which few Wizards know is that his father was Tom Riddle, a Muggle. Severus Snape is also a half-blood (he gave himself the nickname "The Half-Blood Prince"), as his father Tobias Snape was a Muggle. Harry himself is a half-blood, since his pure-blood father, James, married a Muggle-born witch named Lily, and his maternal grandparents were Muggles.

Muggle-born

Muggle-born is the term applied to wizards and witches whose parents were Muggles. J.K. Rowling has said they are second-most common of the three types of blood status wizards, numbering about 25% in the Wizarding community. They are often believed to be descended from Squibs who married Muggles, and the dormant magical gene may resurface after a couple of generations. Harry Potter's mother, Lily Evans, and his best friend, Hermione Granger, were Muggle-born. Unlike children of Wizarding families who get a Hogwarts acceptance letter from an owl, a Hogwarts employee will usually hand-deliver the letter to Muggle-borns in order to meet the Muggle parents and explain.

Supremacists typically believe Muggle-borns to be magically deficient, despite examples to the contrary, such as Hermione and Lily, who are exceptionally skilled in their abilities.

Pureblood supremacists refer to Muggle-borns with the offensive derogatory term Mudblood. Hagrid was shocked to find out that Draco Malfoy uttered the term to Hermione in order to insult and provoke her, since the slur is never used in proper conversations. Hermione, after being tortured by Bellatrix Lestrange, due to her blood status, decided to reclaim use the term "Mudblood" with pride instead of shame in an effort to defuse its value as a slur.

During Voldemort's rule, Muggle-borns were legally required to register with the Muggle-Born Registration Commission, headed by Dolores Umbridge. During this time, the Department of Mysteries claimed that Muggle-borns acquired their magic by stealing magic and wands from "real" wizards. Other wizards and witches rejected this notion, such as Ron Weasley, who asks how such an accusation can be proven. After the fall of Voldemort, Dolores Umbridge is imprisoned in Azkaban, with the other members of the commission either imprisoned or becoming fugitives.

Squibs

Squib is the term applied to a child who is born of magical parents, but who develops no magical abilities. They are considered to be the opposite of Muggle-born wizards/witches. Squib births are rare: the only Squibs noted as such in the books are Argus Filch, Arabella Figg, and Molly Weasley's second cousin, who was an accountant. The Ministry does not require them to be registered as part of the community. Squibs share some things with wizards and they are aware of and comprehend the wizarding World. They can also see Hogwarts and Dementors, which ordinary Muggles cannot. However, according to Ron's Aunt Muriel, the custom with Squibs has been to send them to Muggle schools and encourage them to integrate into the Muggle world, which is "much kinder" than keeping them in the wizarding world where they will always be "second-class". In contrast to most of the wizarding world's acceptance and even respect for Muggles and Muggle-born wizards and witches, it is often considered embarrassing to have a Squib in the family. Neville once said that his family originally thought he was a Squib until his great-uncle Algie dropped him out of a second-story window and Neville bounced down the road. Albus Dumbledore's sister, Ariana, was inaccurately rumoured to be a squib.

Mixed species

Some wizards are the products of unions between humans and magical creatures of more-or-less human intelligence, such as Fleur Delacour and her sister Gabrielle (both quarter veela), Professor Flitwick (a quarter goblin), and Madame Maxime and Hagrid (both half giant). Prejudiced wizards (such as Umbridge) often use the insulting term half-breed to refer to mixed-species wizards and werewolves, or to other beings such as house elves, merpeople and centaurs (who are separate species). The centaurs within the series prefer to exist amongst themselves, with little interaction with humans.