Virtual worlds are networked three-dimensional environments. They simulate physical interaction in three-dimensional spaces and decouple such interaction from geographic constraints. People participate through their avatars, manipulating objects and communicating with others. Virtual worlds open new avenues for education, business, and scientific discovery, and can significantly enhance collaboration in distributed organizations. This requires scalable and secure virtual world systems, complemented by appropriate tools for creating virtual world content. Conventional architectures for virtual world systems were developed for military training and gaming. They do not take advantage of lessons learned from the World Wide Web on building robust and scalable distributed systems. All virtual world systems currently in operation suffer from fundamental scalability and security limitations. No currently available platform can both accommodate dynamic user-modifiable content and scale to millions of concurrent users. This talk will highlight the central challenges in virtual worlds research and describe some preliminary solutions.