For some Muslims, Sufism (Islamic mysticism) is the defining essence of Islam, without which one is left with only a meaningless shell for a religion. For other Muslims, Sufism stands as the satanic antithesis to God's singular, eternal truth. What then is Sufism? And why does it elicit such fervent reactions from its champions and detractors alike? This course proposes to answer these very questions. After an introductory look at various definitions and manifestations of Sufism today and in the past, students begin with an historical survey of the earliest precedents and intellectual currents in the development of Sufism. Students continue with a study of Sufi poetry, terminology, institutions, and rituals. Additionally, students look at key Sufi personalities, both men and women. The final part of the course examines modern Islamic intellectual trends and their effects on debates and discussions within the study and practice of Sufism in the twentieth century. The course returns in the end to contemporary Sufism and the diverse forms that it assumes across the globe today.