Extra-sensory perception (ESP) refers to any phenomena in which one gains information through means other than the recognized physical senses. ESP is therefore a broad category. Though ESP has never been scientifically proven and has numerous critics and skeptics, many laboratory test results have strongly favored the existence of ESP, and many people claim personal experience with it. ESP includes such phenomena as telepathy, out-of-body experiences, clairvoyance or remote viewing, precognition or seeing the future, aura reading, and other instances of intuition and knowledge acquired without the use of the physical senses.

The field of study that deals with ESP, known as parapsychology, was developed by J. B. and Louisa Rhine at Duke University in the 1930s. The Rhines popularized the terms ESP and psi, the hypothetical agent through which the mind experiences ESP. They also introduced the use of Zener cards, designed by psychologist Karl Zener, to test ESP in the laboratory. The Zener cards consist of a deck of 25 cards with five different symbols, and ESP is tested by recording the accuracy of the test subject's guesses about the symbols on cards he or she cannot see.

Though many laboratory experiments suggest that ESP is a real phenomenon, none are definitively conclusive, and many more have been unsuccessful. Critics complain that the experiments favoring ESP were conducted with improper methodology. ESP is, by its nature, difficult to test, since there is little consensus over how it works or what exactly would prove its existence. Few people are truly disinterested when it comes to ESP.

Interestingly, some laboratory tests on ESP found that those who believe in psi are more likely to score above chance when guessing Zener card symbols, while those who do not believe in psi often score below chance. Another interesting result showed people in a relaxed state of mind, such as hypnosis, to make accurate guesses about Zener cards twice as often as chance would indicate, while those who are not hypnotized tend to score around chance.