This chapter describes how purity and defilement were practiced and discussed in diverse cults throughout the Hellenistic and Roman Empires and in contemporary Judaism. There were several types of purity and defilement. The first, a “truce” impurity perception, was temporary and mundane, a defilement occurring when there was an obstruction to the normal order or when categories were mixed up. A second type, the “battle” impurity perception, followed exceptional actions, typically deliberate, such as murder or adultery. Here purification required both punishment by the community and ritual actions, such as sacrifice. A third type became more and more significant in the first centuries CE. This was the defilement of the individual by his or her evil actions and dispositions, conceptualized at times as a “defilement of the soul,” and its purification through asceticism, philosophy, or repentance. Though purity and defilement also featured in Greco-Roman religion, it received an unusually central role in Judaism.