The remit of theology is to study God and all things as they relate to God, as Thomas Aquinas has it. Among ‘all things’, for a theologian with an interest in living creatures, few developments in our understanding of the natural sciences better reward our attention than work on biological mutualism: mutually beneficial cooperation between organisms of different species. Contemporary biology presents us with a vision of interrelation of startling extent and significance. This has, however, received almost no attention from theologians. The essays in this edition of Theology and Science attend to that. Among them, one paper offers a broad summary of the science, written with the non-scientist in mind, which may serve as a prelude for the discussions in the present paper.