The Patronus Charm is a spell that can be used to repel Dementors. Remus Lupin described it as a positive force of hope, happiness and the desire to survive (the things that Dementors feed on) that is projected to make a kind of shield between the caster and the victim. As the Patronus cannot feel despair Dementors cannot affect it.
The Patronus is produced by the incantation "expecto patronum". However to make the spell work you also need to concentrate on a very happy memory. The spell is advanced, above the level of O.W.L.s.
The spell looks like a bright wisp of silvery gas. A corporeal Patronus will drive away Dementors, though it needs powerful magic to cast it. The corporeal Patronus will look different to each witch or wizard who conjures it. The form is that of a creature, reflecting the nature of the caster. Animagi seem to use this same choice of creature to transorm into. A great emotional upheaval can cause the shape of a Patronus to change, for example in incidences of unrequited love the caster's Patronus will take on the form of the nature of man or woman they are in love with instead of their own.
An additional use for the Patronus is for communicating over long distances. It is particularly useful as there can be no doubt as to the sender of the spell. The Patronus will arrive at its intended destination and speak the message with the voice of the caster. This method seems to be mostly used only by the Order of the Phoenix as a secret way of talking to each other. It is possible to produce multiple versions of the same Patronus at once in order to send the message to several people.
 Uses in the books
 Third Book
We first see a Patronus being used in Harry Potter and the Prisoner of Azkaban by Lupin to repel a Dementor that was searching the train on behalf of the Ministry of Magic. As Harry Potter was particularly affected by the presence of Dementors, Professor Lupin agreed to teach Harry how to cast the spell, even though it was magic that Harry should not have learnt for another three years at least. Despite the advanced nature of the magic, Harry was able to cast the gaseous form of the spell in the presence of a Boggart (which for him takes on the shape of a Dementor) at his third attempt.
Harry first conjures a full-body Patronus in the middle of a Quidditch match against Ravenclaw. However the Dementors that Harry thought he was casting the spell at were really Draco Malfoy, Vincent Crabbe, Gregory Goyle and Marcus Flint in disguise as they hoped to distract Harry from the game at a crucial moment. The spell caused them to topple over.
When Harry, Hermione Granger and Sirius Black are attacked by a large number of Dementors near the lake, Harry is unable to cast a Patronus charm powerful enough to repel them. Hermione wasn't able to cast the spell at all (though she had not been taught). Harry then witnesses an incredibly powerful corporeal Patronus Charm that drives them away. This is later revealed to have been cast by Harry himself after he travels back in time with the use of a Time-Turner.
 Fourth Book
During the Third Task of the Triwizard Tournament, Harry comes across what seems to be a Dementor. However when using the Patronus Charm against it, the 'Dementor' stumbled and Harry realised it was really a Boggart.
 Fifth Book
To his immense surprise, Harry and his cousin Dudley were attacked by Dementors in the school holidays. Harry tries twice to cast a full-body Patronus, but only succeeds on his third attempt. Because it is illegal for an underage wizard to use magic outside of school, especially in the presence of a Muggle, Harry is called to a hearing in the Ministry to face the possibility of expulsion. However, Dumbledore is able to convince the Wizengamot that Harry's actions fell under the exceptional circumstances that include saving his own life and that of another who is present. As a result Harry is cleared of the charges against him. It is during the hearing that the term 'corporeal Patronus' is first used, by Amelia Bones who was impressed with Harry's magical ability.
When Harry forms Dumbledore's Army as a secret Defence Against the Dark Arts club, he is able to teach the members (who are of several different ages, some of them young) how to produce the Patronus. Although he does have to warn them that in the actual presence of a Dementor it is not so easy to cast the spell. It was during one of these lessons that they are caught by Dolores Umbridge.
When Dumbledore tells Harry that members of the Order of the Phoenix have a better manner of communicating than by using Umbridge's fire, he is referring to their method of sending messages via Patronus. So Snape must have sent a Patronus to Sirius Black in order to check that Harry's report that he was kidnapped in the Department of Mysteries was false.
 Sixth Book
Nymphadora Tonks uses the spell to summon Hagrid to open the gates to Hogwarts after Harry missed the coaches up to the school and was caught out by the new security. However Severus Snape takes the message instead (as Hagrid was late) and on coming to collect Harry notes that her Patronus has changed form. This was as a result of her unrequited love for Lupin. He taunts her by commenting that she was better off with her old one, saying that the new one looked weak.
 Seventh Book
Kingsley Shacklebolt uses his Lynx Patronus to warn those at the wedding of Bill and Fleur that the Ministry has fallen into Voldemort's hands. Once the trio had run to Number 12, Grimmauld Place Arthur Weasley sent them another message in the same way to tell them that their family was safe.
Severus Snape uses his doe Patronus to lure Harry into a situation where he can be given the Sword of Gryffindor.
Harry casts a Patronus charm in the village of Hogsmeade that sets off a Caterwauling Charm causing several Death Eaters to look for him, Ron and Hermione. However they are saved by Aberforth Dumbledore, who claims that he sent the Patronus in order to put out his cat.
During the Battle of Hogwarts Harry is unable to cast a Patronus to defend himself from Dementors in the face of his own despair. Hermione and Ron's spells aren't powerful enough to repel them, however DA members Luna Lovegood, Ernie Macmillan and Seamus Finnigan use their Patronuses and the effect of them is enough to enable Harry to conjure his stag which really drives the Dementors away.
 Known Patronuses
Ron Weasley - Jack Russell terrior
Albus Dumbledore - Phoenix
Minerva McGonagall - Cat
Kingsley Shacklebolt - Lynx
Dolores Umbridge - Cat
- ↑ 1.0 1.1 J. K. Rowling, PoA 12
- ↑ J. K. Rowling's Official Website, Section: F.A.Q., So how DO the members of the Order of the Phoenix communicate with each other?
- ↑ J. K. Rowling, PoA 5
- ↑ J. K. Rowling, PoA 10
- ↑ J. K. Rowling, PoA 13
- ↑ J. K. Rowling, PoA 20
- ↑ 7.0 7.1 J. K. Rowling, PoA 21
- ↑ J. K. Rowling, GoF 28
- ↑ J. K. Rowling, GoF 31
- ↑ J. K. Rowling, OotP 1
- ↑ J. K. Rowling, OotP 8
- ↑ 12.0 12.1 12.2 J. K. Rowling, OotP 27
- ↑ J. K. Rowling, HBP 8
- ↑ J. K. Rowling, DH 8
- ↑ 15.0 15.1 J. K. Rowling, DH 9
- ↑ 16.0 16.1 J. K. Rowling, DH 28
- ↑ 17.0 17.1 17.2 17.3 J. K. Rowling, DH 32
- ↑ Harry Potter and the Order of the Phoenix (film)