In a clinical treatment context, the importance of hypnotizability is closely akin to the importance of individual differences in response to other treatments, for example, medications, the appreciation of which is crucial for high-quality health care. Hypnosis produces a number of potentially important subjective effects that can be measured, including feelings of hypnotic depth, transferential phenomena like archaic involvement, and other phenomenological states. However, there is fairly wide agreement that a key subjective effect of hypnosis is an alteration of the sense of agency, such that when the hypnotic subject carries out a suggestion, it feels as if it is happening on its own, extra-volitionally. Hypnosis scales are the foundation for laboratory research on hypnosis, and published laboratory research on hypnosis virtually always uses them. The measurement of hypnotizability through standardized hypnosis scales has provided the foundation of modern hypnosis research but has had relatively little impact on clinical practice.