A century ago, William James introduced the study of a class of extraordinary experiences to psychology in The Varieties of Religious Experience. In the years since, psychological and neurophysiological studies have begun to improve our understanding of these mental states; however, the need has recently become more urgent, as psychopharmacological therapies are now being used to induce these experiences in clinical contexts. This dissertation describes the conceptualization, measurement, and experimental manipulation of self-transcendent experiences—defined as transient mental states involving feelings of self-loss and/or connectedness. The first chapter utilizes quantitative forms of computational linguistic analysis (N = 2,389) to identify correlates of these experiences in language. The second chapter introduces a conceptual model that places common psychological constructs with a self-transcendent component (e.g., flow, mindfulness, awe, peak experience, mystical experience) on a theoretical continuum of subjective intensity. The third chapter describes the development and initial validation (across studies N = 1,890) of multi-dimensional psychometric scales to measure self-transcendent experiences and shows that the connection sub-scale—not the self-loss sub-scale—is related to well-being. The fourth chapter describes experimental manipulations of self-transcendent experiences (across studies N = 1,286). This work has implications for informing emerging psychiatric, psychotherapeutic, and well-being interventions.