Psi is the term used by researchers for a variety of demonstrable but elusive psychic phenomena. This collection of essays provides a detailed survey of the evidence for psi at the level of scientific review. Key features of apparent psi phenomena are reviewed, including precognition and remote perception (knowledge of future or distant events that cannot be inferred from present information), presentiment (physiological responses to stimuli that have not yet occurred), the effects of human emotions on globally dispersed machines, the possible impact of local sidereal time on psi performance, and the familiar feeling of knowing who is calling on the phone. Special attention is given to those phenomena that make it difficult for scientists to get a clear understanding of psi. The body of psi research, while complex and frustrating, is shown to contain sufficiently compelling positive evidence to convince the rational open-minded observer that psi is real, and that one or more physical processes probably underlie observed psi phenomena.