When using hypnosis, one person (the subject) is guided by another (the hypnotist) to respond to suggestions for changes in subjective experience, alterations in perception, sensation, emotion, thought or behavior. Hypnosis is often discussed with regards to 'trance' and 'suggestion'. A hypnotic induction is used to encourage the subject to enter a hypnotic 'trance', typically followed by suggestions designed to produce specific experiences. There is considerable debate as to whether hypnosis constitutes an 'altered state of consciousness' and whether such an altered state is necessary for successful response to suggestions. A number of reviews have concluded that hypnosis is an efficacious treatment for pain and irritable bowel syndrome (IBS), and others point towards the efficacy of the use of hypnosis as an adjunct to other forms of therapy.
In the U.K. system of categorizing complementary therapies, hypnotherapy is a Group 2 complementary technique used to complement conventional medicine, which does not purport to embrace diagnostic skills.  The use of hypnosis in treating behavioural disease and dysfunction, principally mental disorders.
State vs. Non-state
A key debate throughout the 20th century was whether hypnosis constitutes an 'altered state of consciousness'. Dietrich (2003) describes characteristics typical of an altered state of consciousness, including: "a sense of timelessness, denial of self, little if any self-reflection and analysis, little emotional content, little abstract thinking, no planning, and a sensation of unity", and aspects of hypnosis has been associated with at least some of these. Brain imaging has revealed characteristic changes in EEG activity following the induction of hypnosis, and changes in the way the brain resolves conflict have been reported (Egner et al, 2005).
In contrast, it is a common finding that responses to suggestions are similar in the presence and absence of a hypnotic 'state', leading some to question the necessity of an altered state when attempting to explain hypnotic phenomenon (Kirsch & Braffman, 2001).
- Select Committee appointed to consider Science and Technology, U.K. Parliament (21 November 2000), Chapter 2: Disciplines examined, Definitions of the Various CAM Therapies, Complementary and Alternative Medicine