This article will provide an analysis of the various ways in which blood appears in the Historia Albigensis of the Cistercian historian, Pierre des Vaux-de-Cernay. This history, written between 1213 and 1218, is well-known for both its narrative of the first stage of the crusade against the heretics of Languedoc and for its eulogistic treatment of Simon de Montfort. Historians have also mined this chronicle for information about the various heretical sects purported to exist in the Languedoc. Less attention has been paid to the narrative means by which Pierre linked heresy and the activities of heretics, the unique nature of the crusade, and the enormous violence which characterized the first phase of the crusade. I suggest that blood provided a useful set of concepts with which Pierre could both construct an understanding of the danger of heresy and argue for an appropriate means to deal with that danger. In the Historia, blood appears as the corruptible fluid that runs between and connects generations of heretics; as a miraculous fluid which portends the coming of the crusaders; and as a salvific fluid which destroys evil and creates martyrs. This article will explore these three manifestations of blood in the context of the Historia's narrative structures.