Abstract. Many of the most interesting, and most debated, questions in the philosophy of religion are questions about compatibility. Most arguments against the existence of a necessarily existing, omniscient, omnibenevolent, and omnipotent being take the form of arguments from inconsistency of the existence of such a being with various apparent features of the world, such as the existence of evil or of human free will. In response, believers in the existence of such a being have sought to show that the existence of such a being is, in fact, compatible with such features of the world. However, the fact that the proposition that God exists is necessary if possible introduces some underappreciated difficulties for these arguments. I illustrate these difficulties by consideration of Warfield's argument for the compatibility of free will and divine foreknowledge, and Plantinga's argument that God's existence is compatible with the existence of evil. I argue that both arguments fail to establish their intended conclusions, before turning to the question of what compatibility arguments of this sort might show.