The book presents research in understanding how to make Virtual Reality (VR) applications more enjoyable and realistic. The author studies this in relation not just to VR games for entertainment, but also for other applications such as serious games, which are used for education and training. To make VR games better, the work examines topics like the content in the game, the context in which its played, and human factors like one age, and previous experience with VR. By understanding these factors, the author creates guidelines to make VR games more engaging and user-friendly. The primary purpose of this work is to evaluate and identify different influences on User Experience (UX) for VR serious games, intending to narrow the research gap between Influencing Factors (IFs), UX, and design guidelines for VR serious games. With eight user studies and five different VR serious games developed, different influences and links between those factors and UX components are investigated.