Skeptics tend to regard hypnosis as deceptive magic acts. At its worst, hypnosis may be entirely staged with apparent "volunteers" eager to show off their acting skills. At its best, however, people can actually be induced to go into a trance-like state where they're more susceptible to suggestions.
Increasing evidence suggests hypnosis isn't just an illusion. Getting people in a trance-like state has been found to alter brain activity. In a brain scan of people under hypnosis, scientists found their brain activities were significantly different than when they were at rest.
Furthermore, it's been demonstrated that people under hypnosis can perform extraordinary tasks that they would otherwise not be able to do. For example, a patient's ability to recall distant, suppressed memories under hypnosis is a valuable therapy that may help people work through their psychological barriers. In another case, a famous experiment demonstrated that post-hypnosis, participants were able to effortlessly ace the difficult Stroop task. These studies show quantifiable evidence that hypnosis is a real neurobiological phenomenon, but that more research is needed to fully understand it.