"Presence", the sense of "being there" in a mediated environment, has been applied to describe the user experience when interacting with advanced media interfaces such as virtual environments. Why examine the concept of presence? There are compelling practical and theoretical reasons. The definition of the construct of presence is of particular interest today because it has the potential relevance for the design and evaluation of a broad range of interactive and non-interactive media and applications in several areas such as medicine, entertainment, education and training. An enhanced sense of presence plays an important role in technologies such as the video telephone, high definition television (HDTV), 3G portable phones, home and arcade video games, the World Wide Web, and more. The book examines the different facets of the concept of presence. It begins by noting practical and theoretical reasons for studying this concept. Different theories of presence are identified and a detailed description of the concepts included in these conceptualizations is presented. Existing research and about the factors that encourage or discourage a sense of presence in media users as well as the physiological and psychological effects of presence are then outlined. Finally, suggestions concerning possible evolutions of this concept allowed by the diffusion of ambient intelligence and 3G portable communication are presented.