Quantum change-the subject of this issue of In Session-refers to sudden, dramatic, and enduring transformations that affect a broad range of personal emotion, cognition, and behavior. This phenomenon has been described since the beginnings of psychology, most notably by William James in Varieties of Religious Experience. Quantum changes occur both within and (mostly) outside the context of psychotherapy and show certain common features in both process and content. This introduction provides an overview of the phenomenon of quantum change, describing its essential features, two qualitative subtypes, antecedents, course, and the nature of the enduring changes that ensue.

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