This article deals with a complex family of theological viewpoints, collected under the name ‘ground-of-being’ theologies. It discusses their relationship to various forms of theism, their shared themes, and their connections with the natural sciences. Ground-of-being theologies have in common two important negations: they deny that ultimate reality is a determinate entity, and they deny that the universe is ontologically self-explanatory. The positive formulations of ground-of-being theologies vary. Ground-of-being theologies are important, because their denial that ultimate reality is a determinate entity establishes a valuable theological contrast with determinate entity theisms such as personal theism and process theism – two ideas of God prominent both in modern theology and in the contemporary science–religion dialogue.