As in many areas of science, those professionals who specialize in hypnosis, either as an academic subject or as a clinical procedure, are occasionally required to provide lawyers or the prosecuting authorities with the benefit of their specialized knowledge and expertise. This article reviews the circumstances in which experts in hypnosis are asked to provide this service and it examines how current knowledge and understanding of hypnosis, and human psychology in general, informs this work. It considers both criminal and civil cases. In most of the casework that this article describes, the expert witness is asked to address certain questions about hypnosis and to prepare a report, which he or she sends to the instructing party. Its principal focus is to understand the nature of hypnosis from a rational, scientific perspective and how this can inform the answers to questions that lawyers and the police ask about hypnosis.