MOO stands for "Multi-user domain, Object-Oriented." Early multi-user domains, or "MUDs," began as net-based dungeons-and-dragons type games, but MOOs have evolved from these origins to become some of cyberspace's most fascinating and engaging online communities. MOOs are social environments in a text-based virtual reality where people gather to chat with friends, meet new people, and help build the MOO. Users connect from anywhere in the world and communicate with one another in real time. Users can create rooms, objects, and programs that recreate in text anything the user might imagine. Educational MOOs have an academic theme and use a variety of MOO communication tools such as internal e-mail, newspapers, documents, blackboards, and classrooms to accommodate a variety of teaching styles. Teachers can use these tools in harmony with the goals for the class while exploiting the nature of the MOO as a student-centered learning environment. The paper provides: names, descriptions, and sites of some educational MOOs suitable for learners of high school age and older; a description of user roles on MOOs--guests, builders, programmers, wizards, and archwizards; difficulties teachers may face when using a MOO; information on how to get started; and guidelines for ensuring a positive experience for both teachers and students. Resources for more information on educational MOOs are provided. (SWC)