Accessed by entering a trapdoor in the forbidden corridor on the third floor, and protected by a gauntlet of seven magical challenges set up by the teachers.

A giant three-headed dog named Fluffy placed specially to guard the trapdoor by Hagrid.
Devil's Snare, grown by Professor Sprout.
A room containing dozens of keys, charmed by Flitwick to sprout wings and fly near the ceiling. One of these keys will unlock the door to the next section. However, in the film adaptation, the keys attack the seeker of the Stone.
A large chessboard with an army of large chessmen, transfigured by McGonagall. To continue to the door on the opposite side, the person in question must beat the chessmen at a game of wizards' chess where the player must risk his life if he loses. Ron and Professor Quirrell are the only wizards to win the game of wizards' chess.
A room with a large troll inside. This is Quirrell's challenge. In the book, Quirrell had knocked out his own troll to get to the last room and thus the trio did not have to fight it; in the film, it does not appear, but it appears in the PS1 and Game Boy Color version of the game.
A series of potions, brewed by Snape. A logical riddle, not magic, has to be solved. There are two doors, blocked by fire. One potion will allow the person to exit the way he or she arrived, another will allow him or her to continue to the next chamber, two are nettle wine, and the other three are poison. This challenge does not appear in the film, but does in the video game adaptation.
The Mirror of Erised can be found in the final chamber, further enchanted by Dumbledore to bestow the Philosopher's Stone upon a seeker only hoping to acquire the stone but not use it for selfish means.