Perfect being theism is a version of theism that says that God is the greatest possible being. Although perfect being theism is the most common form of monotheism in the Judeo-Christian-Islamic tradition, its truth has been disputed by philosophers and theologians for centuries. Maximal God proposes a new, game-changing defence of perfect being theism by developing what the book calls the ‘maximal concept of God’. Perfect being theists typically maintain that God is an omniscient, omnipotent, and omnibenevolent being but, according to Maximal God, God should be understood rather as a being that has the maximal consistent set of knowledge, power, and benevolence. The book argues that once we accept the maximal concept, we can establish perfect being theism on two grounds. First, we can refute nearly all existing arguments against perfect being theism simultaneously. Second, we can construct a novel, strengthened version of the modal ontological argument for perfect being theism. The book concludes that the maximal God concept provides the basis for a unified defence of perfect being theism that is highly effective and economical.