Max Weber argued that medieval religious movements were an important source for the distinctive rationality of Western civilization. He intended to study this theme but died before he could do so. In Schools of Asceticism, Lutz Kaelber builds on Weber's ideas by presenting a fresh analysis of asceticism in orthodox and heretical religious groups in the Middle Ages. Based on extensive research using primary and secondary sources, this book bridges the disciplines of comparative and historical sociology, medieval history, and religious studies.