The virtual simulation of Radiotherapy Treatment Planning (RTP) replaces the real Simulator machine with a virtual one. In this book, two major planning concepts, visualized and collaborative planning are presented. The interactive volume visualization techniques related to the visualized planning are introduced in detail, including interactive and realistic Digital Reconstruction Radiography (DRR), interactive Maximum Intensity Projection (MIP), multi-volume rendering, vessel feature enhancement, and volume interaction techniques. In contrast, collaborative planning links physicians distributed around the world via networks, and allows simultaneously teamwork on the same treatment plan in real-time. Based on these two concepts, a collaborative virtual simulation environment for RTP, called EU-VIRTUOSO, is built up as an application example.