 There is a host of arguments under this title; version of the argument works, then it can be deployed using only the concept of God as maximally excellent and some modal principles of inference, that is, principles concerning possibility and necessity. The argument need not resist all empirical support, however, as shall be indicated. The focus of the argument is the thesis that, if there is a God, then God’s existence is necessary. In other words, God’s existence is not contingent—God is not the sort of being that just happens to exist or not exist. That necessary existence is built into the concept of God can be supported by appealing to the way God is conceived in Jewish, Christian, and Islamic traditions. This would involve some a posteriori, empirical research into the way God is thought of in these traditions. Alternatively, a defender of the ontological argument might hope to convince others that the concept of God is the concept of a being that exists necessarily by beginning with the idea of a maximally perfect being. If there were a maximally perfect being, what would it be like? It has been argued that among its array of great-making qualities (omniscience and omnipotence) would be necessary existence. Once fully articulated, it can be argued that a maximally perfect being which existed necessarily could be called “God”. For an interesting, recent treatment of the relationship between the concept of there being a necessarily existing being and there being a God, see Necessary Existence by Alexander Pruss and Joshua Rasmussen (2018: chapters one to three).