World of Warcraft also features instances. Instances, or "Instanced Dungeons", are dungeons that have been designed and tailored specifically for certain sizes of groups and are duplicated for each group that enters it. This way a party can experience a dungeon without interference from other World of Warcraft players, and cannot zerg it by bringing in outside help to overwhelm the opposition. Instances will allow only a certain maximum group size to enter, ranging from 5-man groups in some small instances, to 40. As of mid-2005 there are three instances which allow for a maximum of 40 people: Onyxia's Lair, The Molten Core, and Blackwing Lair. These are the toughest areas in the game and success in these three instances is hardly possible without great organization and good equipment. The recently added World of Warcraft Zul'Gurub instance allows for a maximum of 20 people and offers a more casual gameplay experience.

Typically, World of Warcraft instanced dungeons are more elaborate and require more steps to be taken in a particular order for successful completion than are necessary in the main world. For example, requiring that a large group of enemies be defeated so that one can pull a lever to rotate a bridge in order to access a new section of the map makes sense when one group is traversing the dungeon linearly; it makes far less sense in public areas where hundreds of other World of Warcraft players might be in the same area. Therefore, instanced dungeons tend to be much more interesting than normal areas, in which quest requirements are very simple (e.g. kill a certain number of enemies, or collect a certain number of objects).

In December 2010 World of Warcraft: Cataclysm was released. At this time the subscription base had reached 12 million players world wide.