The Shield Charm is a spell used to repell a minor curse or jinx during a duel, using the incantation Protego.
 Effects and Limitations
The Shield Charm creates a temporary, invisible wall around the caster, which should deflect spells. The strength and cover this spell provides depends on how well it is cast.
 Uses in the books
 Second Book
During the Duelling Club formed by new Defence Against the Dark Arts professor, Gilderoy Lockhart, he possibly intends to teach Harry Potter how to cast a Shield Charm during a duel between him and Draco Malfoy, after being prompted by Severus Snape to teach them to block unfriendly spells. However all Lockhart succedes in doing is dropping his wand.
 Fourth Book
Harry Potter learns how to use the Shield Charm along with his friends, Hermione Granger and Ron Weasley, in the run up to the Third Task in the Triwizard Tournament when facing unknown challenges. Harry struggled to learn this particular spell. During practicing this charm Hermione manages to find a weak point in Harry's shield and hits him with the Jelly-Legs Jinx, to which she didn't know the counter-jinx and had to look it up.
 Fifth Book
Harry teaches the Shield Charm to Dumbledore's Army during his fifth year at Hogwarts. During this time Neville Longbottom's ability had increased rapidly and he was the quickest to master the spell after Hermione.
When six members of Dumbledore's Army break into the Department of Mysteries in the Ministry of Magic, Harry manages to predict and block Death Eater Bellatrix Lestrange's attempt to Summon the prophecy he was holding. She mockingly responds that Harry knows how to play.
 Sixth Book
During Severus Snape's first lesson as Defence Against the Dark Arts teacher, he teaches the class how to cast spells silently. The aim was for one student to cast a silent jinx at their opponant, who would create a Shield Charm silently to deflect it. Only those of the class who had been taught by Harry in Dumbledore's Army had ever used the spell before, however even with that advantage it was a difficult lesson. When Snape saw that Ron was unable to jinx Harry, he quickly took over and sent a spell so fast at Harry that he only had time to react reflexively, yelling the incantation. The resulting Shield Charm was so powerful it knocked Snape backwards.
 Final Book
During the Battle of Hogwarts, after Harry had through chance faked his own death, he was able to create Shield Charms protecting those who Lord Voldemort was trying to curse, despite Voldemort using the Elder Wand, which was reputedly Unbeatable. This was a demonstration of how Voldemort's spells were no longer holding, how he didn't yet have mastery of the wand. Harry was able to create a large Shield Charm in the middle of the Great Hall, finally setting the scene for a one-on-one duel between himself and Voldemort.