Lupin was bitten by a werewolf when he was very young. His parents were both upset by this, because it meant he could not go to Hogwarts School of Witchcraft and Wizardry. However, when Albus Dumbledore became headmaster, he let Lupin go to Hogwarts. He had a tunnel built so that Lupin could sneak out of the school during the times when he became a werewolf. This tunnel had the Whomping Willow (a violent tree that attacks those who get near it) built over it, so nobody could get to him by mistake. The tunnel led to the Shrieking Shack, a building in the wizarding village of Hogsmeade. When Lupin was in the Shrieking Shack as a werewolf, he had no humans to bite, so he often bit himself. These bites led to cries of pain that led the villagers to think that the building was haunted.
During his time at Hogwarts, he became good friends with three other boys his age: James Potter, Sirius Black, and Peter Pettigrew. These four came to be known as the Marauders, infamous for their mischievous antics. They knew Hogwarts and the grounds like the back of their hands, and actually created a magical map that showed the location of everyone on the grounds and in the building. The map also revealed the many secret passages hidden in the school. His friends knew of his lycanthropy, but did not seem to mind. They simply referred to it as his "furry little problem". Later on, they even became Animagi (a person that can turn into an animal) so that they could accompany him on the full moon and make his transformations more enjoyable.
On the 31st of October, Remus lost all of his friends at once, through betrayal, jail and death.
He was a wizard during Voldemort's first and second rises to power. He was member of the First and Second Order of the Phoenix and taught as the Defence Against the Dark Arts professor in the 1993-94 school year at Hogwarts. He quit at the end of the year when Severus Snape let it slip to students that he was a werewolf.
He was killed along with his wife Nymphadora Tonks in the Final Battle for Hogwarts. They were survived by their son Teddy Lupin, who was left in the care of Tonks' mother, Andromeda Tonks, and his godfather, Harry Potter.