Past-life regression, or hypnotic regression, is a kind of therapy in which a person is placed in a hypnotic trance during which they remember key moments from past lives. This type of therapy can be undergone for a variety of reasons, including resolving trauma or simple spiritual curiosity. In Many Lives, Many Masters, Weiss begins to treat Catherine using regular hypnosis, which is “just a state of focused attention” and has “nothing mysterious about it” (24), but when Catherine begins to re-live memories from past lives under hypnosis, Weiss hones the practice and conducts long term past-life regression sessions with Catherine. After Catherine, Weiss conducted past-life regression on thousands of patients who similarly experienced memories from ancient lifetimes.

Reincarnation is a concept present in numerous religious traditions which state that life, in the form of a spirit, soul, or some other non-physical essence transcends biological death and is reborn in another form in another life. This is a process that has been going on since time began and will continue perhaps forever. Brian Weiss was originally skeptical of the concept of reincarnation, but his experiences conducting past-life regression with Catherine forced him to rethink his skepticism.