Salafism is a theological movement whose radical wing is today affiliated with al-Qa'ida and the Islamic State, but which draws on precedents stretching back to the medieval theology of Ibn Taymiyya. This innovative study focuses on the concept of theonomy in salafi thought: the tenet that rule by God's law is an essential component of faith, and the corresponding notion that other forms of rule based on human legislation are inherently polytheistic and thereby illegitimate. It is this tenet which furnishes radical militants with their principal casus belli against ruling regimes in the Muslim world. In this book, Daniel Lav details the intellectual grounding for modern salafi theonomy in Ibn Taymiyya's doctrine of tawhid and the writings of the early Wahhabi movement, in addition to the twentieth-century thought of Abu al-A'la Mawdudi and Sayyid Qutb, while drawing on insights from comparative political theology to analyze this key school of thought.