The goal of this closely reasoned study is to explain why, in Priestly texts of the Hebrew Bible, the verb _kipper_, traditionally translated 'atone', means the way of dealing both with sin and with impurity-which might seem very different things. Sklar's first key conclusion is that when the context is sin, certain sins also pollute; so 'atonement' may include some element of _purification_. His second conclusion is that, when the context is impurity, and _kipper_ means not 'atone' but 'effect purgation', impurity also _endangers_; so _kipper_ can include some element of _ransoming_. In fact, sin and impurity, while distinct categories in themselves, have this in common: each of them requires both ransoming and purification. It is for this reason that _kipper_ can be used in both settings. This benchmark study concludes with a careful examination of the famous sentence of Leviticus 17.11 that 'blood makes atonement' (_kipper_) and explains how, in the Priestly ideology, blood sacrifice was able to accomplish both ransom and purification.