Researchers at UCLA's Departments of Architecture and Urban Planning are using state-of-the-art visual simulation technology to build virtual worlds that can be used by planners, designers, and community groups to visualize and evaluate proposed changes and new developments in the urban environment. This paper focuses on the modeling methodology that has evolved for efficiently creating three-dimensional (3D) virtual urban worlds. It is illustrated with reference to actual projects that have recently been completed or are currently under development.