Sense of presence is one of the most interesting phenomena that enriches users’ experiences of interacting with any type of system. It allows users to be there (Schloerb & Sheridan, 1995) and to perceive the virtual world as another world in which they really exist. The interest in presence phenomenon is not novel (Gerrig, 1993), but it has grown lately due to the advent of virtual reality (VR) technology. The specific characteristics of virtual environments (VEs) transform them into suitable experimental testbeds for studies in various research areas. This also resuscitated the interest in presence, and much work has focused on the development of a theoretical body of knowledge and on a whole set of experimental studies aimed at understanding, explaining, measuring, or predicting presence. All of these efforts have been made to increase the understanding of how presence can be manipulated within the VEs, particularly within the application areas where presence potential has been acknowledged. Probably one of the most important reasons motivating presence research is the relationship it holds with task performance. This debatable relationship together with the more obvious one between presence and user satisfaction suggest that presence may play an important role in the perceived system usability. Since presence may act as a catalyst for the learning potential of VEs, it can be harnessed for the training and transfer of skills (Mantovani & Castelnuovo, 1998; Schank, 1997). The potential of presence to increase the pervasive power of the delivered content motivates research on presence impact on e-marketing and advertising (Grigorovici, 2003). Another promising application area for presence research is within the realm of cognitive therapy of phobias (Strickland et al., 1997). The highly subjective nature of presence continues to challenge researchers to find appropriate methodologies and instruments for measuring it. This is reflected in the ongoing theoretical work of conceptualizing a sense of presence. The difficulties related to investigating presence led to a large set of definitions and measuring tools. The purpose of this article is to introduce the concept of presence. The first section offers some conceptual delimitations related to presence construct. The second section describes its main determinants along two dimensions (i.e., technological factors and human factors). The third section addresses the challenges of measuring presence, offering also an overview of the main methods, tools, and instruments developed for assessing it. The fourth section presents the complex relationship between presence and task performance.