This book adheres to the vision that in the future compelling user experiences will be key differentiating benefits of products and services. Evaluating the user experience plays a central role, not only during the design process, but also during regular usage: for instance a video recorder that recommends TV programs that fit your current mood, a product that measures your current level of relaxation and produces advice on how to balance your life, or a module that alerts a factory operator when he is getting drowsy. Such systems are required to assess and interpret user experiences (almost) in real-time, and that is exactly what this book is about. How to achieve this? What are potential applications of psychophysiological measurements? Are real-time assessments based on monitoring of user behavior possible? If so, which elements are critical? Are behavioral aspects important? Which technology can be used? How important are intra-individual differences? What can we learn from products already on the market? The book gathers a group of invited authors from different backgrounds, such as technology, academy and business. This is a mosaic of their work, and that of Philips Research, in the assessment of user experience, covering the full range from academic research to commercial propositions.