Islamist terrorists have had a large influence on U.S. foreign and domestic policy for more than twenty years, and yet policy makers, legal practitioners, and the public know very little about what motivates these violent extremist organizations. A primary unifying principle among the various Islamist terrorist groups is their desire to return to a religiously ordered State, justified and based upon their interpretation of the Shari’a, or Islamic law. This article explores the Islamist terrorist interpretation of Shari’a law and how it generally contradicts that of mainstream Islamic scholars. The article begins with a review of the primary and secondary sources of Islamic law and the attendant controversies. It then defines jihad, explores the Islamic law on authorizing a just war, and highlights how terrorist groups selectively interpret the primary sources to justify their ideology. Next, it explains the divergent views on the law concerning treatment of noncombatants, suicide attacks, treatment of prisoners, and cessation of hostilities. The final section further defines the background, principles, and goals of Islamist terrorist organizations, using al-Qaeda and associated Islamist terrorist groups as an example. It concludes by summarizing the divergent interpretations of Shari’a, and makes modest suggestions on steps that can be taken to alleviate the influence of these terrorist groups.