Virtual worlds like Second Life and World of Warcraft have grown hugely popular, with millions of active users each month. These worlds allow users to create avatars and interact in virtual environments. Some brands have experimented with marketing in these worlds, with varying degrees of success. Researchers have also used virtual worlds to study real-world problems like epidemic spread. While still in their infancy, virtual worlds could eventually become a mainstream form of digital communication, so some marketers recommend experimenting respectfully to understand user behaviors.