Spiritualism serves as an appropriate segue and “case study” for introducing this volume and the questions its authors raise around “mysticism and the margins.” But first: what is “mysticism”? And what might its “margins” entail? This hotly contested category and its boundary demarcations make up the subject matter of these essays. Influentially defined by William James (1842–1910) in his Varieties of Religious Experience (1902) as forms of consciousness that are ineffable, noetic, transient, and passive, “mysticism” would undergo veritable scholarly warfare throughout the middle of the twentieth century through to the present day. Often at the “front lines” of this intellectual battlefield have been notions of (a)historicity, theological authority, subjectivity, gender, and “authenticity”—in other words, who “counts” as a mystic and what qualifies as “mysticism” in general.