To improve the visualization of large 3D landscapes and city models in a network environment, the authors use two different types of hierarchical level-of-detail models for terrain and groups of buildings. They also leverage the models to implement progressive streaming in both client-server and peer-to-peer network architectures. Visualizing large photorealistic 3D landscapes and city models has received significant attention over the last 10 years in the computer graphics community. More recently, the availability of broadband Internet access and Web-based visualization techniques is paving the way for large-scale 3D landscapes and city models for a variety of professional and mass-market services. To make such services appealing to a large audience, these 3D models must reach a sufficient level of realism and accuracy. Many solutions are now available to automatically generate 3D models of huge urban environments. Geographic information system (GIS) databases (such as terrain elevation grids, orthophotographs, and a building's footprint and height) provide a good basis for generating such models at affordable cost with minimum human intervention. In the future, the accuracy and realism of 3D city models will be improved thanks to recent progress in data capturing techniques.