The term Salafism is misunderstood by academic scholars, in part because it is contested among Muslim scholars and groups, who often use the term loosely and even misleadingly. This paper will provide a definition for Salafism, arguing that understood theologically it has a long pedigree in Islamic sectarian history, one that dates back to the medieval period. Salafis adhere to a well-defined creed and have formed a distinctive interpretive community. They reject the beliefs of other Muslim groups such as the Ash`aris and the various Shii sects, among others, and they do so in order to draw a boundary around the community of believers which they defend vigilantly. This process of constantly attacking others is necessary for reinforcing the bonds and identity of the Salafi community, and is a distinctive feature of Salafism. The modern descendants of the Salafis are the Wahhabis but also members of the Ahl al-Hadith movements in South Asia, Yemen, Egypt and elsewhere in the world. Some of these movements predate the Wahhabi movement. Moving away from theology, the consensus among Salafis begins to fray on matters of law and the question of the authority of the established schools (madhhabs). And when it comes to political affiliation and action, the Salafis are profoundly divided and cannot be considered members of a single group. The proof that I will adduce for the importance of theology will come from the long-history of Salafi attacks against the Shiis—from Ibn Taymiyya’s treatise entitled K. Minhaj ahl al-sunna al-nabawiyya to Nasir al-Fahd’s Risala fi mashru`iyyat al-ighlaz `ala al-Rawafid. This is a little studied yet important polemic and this paper will provide a roadmap for the anti-Shii discourse among the Salafis. It will become clear that under the vehement criticism by the Salafis of various Shii practices (e.g., cursing certain companions, visiting the graves of the Imams, etc.) lies a profound difference over theological tenets, one so great as to lead to mutual anathemization and to the potential for violence, as seen in Iraq, Pakistan and elsewhere. Finally, I hope to show that those Muslims who have claimed the mantle of Salafism have invariably had to adopt its theology, part of which involves adopting anti-Shii views. This is the case for certain members of the Muslim Brotherhood who wish to be Salafis as it is for members of al-Qaeda (e.g., Ayman al-Zawahiri) who have not been raised with these beliefs.