Virtual worlds have become very popular in recent years, with trends towards larger worlds and more user-generated content. The growth of 3D content in virtual worlds will make its real-time streaming (or 3D streaming) increasingly attractive for developers. To meet the demands of a large user base economically, peer-to-peer (P2P) content delivery holds the promise for a paradigm shift in how future virtual worlds will be deployed and used. This book defines both the problem and solution spaces for P2P 3D streaming, by outlining its requirements, challenges, and categorizing existing proposals. The first framework for P2P 3D streaming (Flowing Level-of-Details, or FLoD) is described for readers to understand the opportunities and difficulties of such an approach. Open questions are also identified to facilitate the design of future systems. This book is a useful introduction to researchers and developers who want to gain a basic understanding about the new landscape for virtual worlds.