Muggles' Guide to Harry Potter/Places/Room of Requirement
|Muggles' Guide to Harry Potter - Place|
|Room of Requirement|
|Location||Hogwarts, seventh floor|
|First Appearance||Harry Potter and the Goblet of Fire|
General Overview 
The Room of Requirement is a room on the seventh floor of Hogwarts Castle that appears when someone is in very great need of it, and contains what that individual needs at that time.
Extended Description 
The Room of Requirement is located on Hogwarts' seventh floor, across from the tapestry of Barnabas the Barmy.
To "open it" you must walk past the area of the door three times, thinking of what you need. For example: Say you need a place to study. You would walk past the area of the door three times thinking, "I need a place to study." After the third pass, the door will appear and you may enter to find everything you need inside, books, a study surface, and possibly your choice of studying music.
The Room first appears in a passing comment by Professor Dumbledore at the Yule Ball in Harry Potter and the Goblet of Fire. He mentions that he had once found a room full of chamber pots, but has been unable to find it again since. Having learned how the Room works, we may safely assume that he had been walking back and forth in that hall, in need of a bathroom.
When Harry needs a secret place for Dumbledore's Army to meet, Dobby the house elf tells him about this room, describing it as, "A room that a person can only enter when they have real need of it. Sometimes it is there and sometimes it is not, but when it appears, it is always equipped for the seeker's needs." Dobby also called it the "Come and Go Room." Dobby said he had used it as a place to let Winky sleep off the effects of Butterbeer. Harry proposed using it as a meeting place for Dumbledore's Army in Harry Potter and the Order of the Phoenix, exaggerating Dumbledore's earlier mention of it as a way of quieting Hermione's worries about its suitability. All Hermione's fears are allayed when she actually enters the room, and finds it equipped with a complete library of anti-Jinx textbooks. When Fred and George see it, they comment that they had used to hide from Filch, but it had then been no larger than a broom closet. Harry, and we readers, know that the Room took that aspect because that was what the Twins had needed at the time.
In Harry Potter and the Half-Blood Prince, Draco Malfoy uses this room to repair the Vanishing Cabinet. Harry unknowingly uses the same aspect of the room to hide his potions book. Draco's secret activities are discovered when Professor Trelawney, who has been using it to hide her sherry, finds it already occupied by someone who roughly throws her back out into the hallway. The repaired cabinet is used by Death Eaters, who are entering Hogwarts with Draco's guidance to provide cover, while Draco tries to murder Dumbledore at Voldemort's orders.
In Harry Potter and the Deathly Hallows, the revived Dumbledore's Army are using the Room of Requirement as a hiding place. It ends up being quite comfortable, and even, when required, creates a tunnel that ends in an upstairs room at the Hog's Head Inn. Additionally, at Neville's request, it changes its entry point at random, so that guerrilla missions issuing from the Room of Requirement don't always start at the same point in the castle.
Near the end of Book 7, Harry causes the room to revert to its junk warehouse state in order to locate and reclaim Rowena Ravenclaw's lost diadem, which has been made into a Horcrux. He is ambushed in that room by Draco, Crabbe, and Goyle. During their battle, Crabbe sets Fiendfyre, which consumes the contents of the room, almost including Harry, Ron, Hermione, Draco, and Goyle, and does consume Crabbe.
Initially it appears that this room could be all things to all people, always having exactly what is needed for the efforts of the day. It appears, though, that it has a number of standard configurations, as Professor Trelawney (who wants to hide her sherry), Harry (who needs a place to hide his textbook), and Draco (who needs a place to hide while he works on the vanishing cabinet), all end up in the same room, a vast warehouse of broken and hidden stuff. This is actually to be expected; for it to be useful as a meeting place, everyone who wants to meet there must be able to enter the same room, which they do by wanting the same sort of thing. The reason that Harry is unable to enter the room when Draco is in there working is that Draco, being in the room, has locked it into its "junk warehouse" state, and as Harry does not think he is looking for a junk warehouse, he is never able to enter it while Draco is there.
It should be noted that this room is not static; when Harry is first leading class there, he discovers that what he really needs is a whistle, and immediately discovers one on the shelf beside him. In Harry Potter and the Deathly Hallows, Neville, then resident in the Room of Requirement, is able to convince it to provide him a great deal of creature comforts, though it is unable to produce food; this is why it creates a tunnel to the Hog's Head, to allow Neville to get the food he needs from there. The requirement to think of what you need three times apparently only extends to the door of the room; once within the room, a single thought, if expressed urgently enough, seems to be sufficient. Neville, we are told, is a master at expressing these thoughts.
Greater Picture 
One thing that is mentioned in our discussion of this room, in the article on chapter 27 of Harry Potter and the Order of the Phoenix, is how Draco came to be aware of the existence of this room. It is certain that he would never have learned of its existence from the House Elves, as Harry did, and it is unlikely that he would be wandering around the Seventh Floor looking to hide something, so was unlikely to stumble over it accidentally. While this is never confirmed in the books, it is most likely that Draco was told how to get into the Room of Requirement by Voldemort. Voldemort clearly knew about the junk warehouse aspect of the Room, as he hid a Horcrux, Ravenclaw's Diadem, there. It is this same aspect of the Room that Draco used to work on his Vanishing Cabinet. If Draco had wanted a place to be his workshop, and if he had been aware of the multiple nature of the Room, he would have asked for, and gotten, a well-equipped magical workshop and as a result would have been able to complete his task much more quickly. This is why it is critical to the story of Harry Potter and the Half-Blood Prince that it be Pansy, rather than Draco, who retrieves the membership list of Dumbledore's Army from the room in Harry Potter and the Order of the Phoenix; if Draco had been made aware of the multi-aspect nature of the Room, by seeing it first as a Defence Against the Dark Arts classroom and later as a junk warehouse, he very likely would have understood that it could be better matched to what he needed. As it was, the fact that he only uses the junk warehouse aspect of it would indicate that he received word of it from someone who only knew about that aspect, likely Voldemort.
The many interconnections within the story are a hallmark of this author's work. We note that the Room of Requirement was first mentioned in book 4, and played major roles in each successive book; we also note that the nature of the Room, and its ability to be only one thing at a time, were major plot points in book 6. From this, we believe that the author must have worked out the nature of the Room in significant detail by the beginning of her writing of book 5, so as to not accidentally prevent its use by Draco in book 6 or Neville and Harry in book 7.
As mentioned, the Junk Warehouse aspect of the Room of Requirement is destroyed near the end of Book 7 by Fiendfyre. It is possible that this completely destroys all aspects of the Room, but we cannot be certain as no character attempts to re-enter the room once it has been closed on the Fiendfyre. It is safe to assume that all the material hidden in the junk warehouse has been destroyed, and that the warehouse, as a result, would be much smaller if it were re-opened. It is equally safe to assume that any artifacts left in the warehouse, and previously appearing in other aspects of the Room, are now also lost. Whether the Fiendfyre, which we are told is extraordinary magic, being one of only a handful of things that are capable of destroying Horcruxes, would affect the operation of the Room of Requirement, must remain uncertain.