This study of the literary relationship between Charles Williams and C. S. Lewis during the years 1936-1945 focuses on the theme of 'co-inherence' at the centre of their friendship. The idea of 'co-inherence' has long been recognized as an important contribution of Williams to theology, and had significant influence on the thought of Lewis. This account of the two writers' conviction that human persons 'inhere' or 'dwell' both in each other and in the triune God reveals many inter-relationships between their writings that would otherwise be missed. It also shows up profound differences between their world-views, and a gradual, though incomplete, convergence onto common ground. Exploring the idea of co-inherence throws light on the fictional worlds they created, as well as on their treatment (whether together or separately) of a wide range of theological and literary subjects: the Arthurian tradition, the poetry of William Blake and Thomas Traherne, the theology of Karl Barth, the nature of human and divine love, and the doctrine of the Trinity. This study draws for the first time on transcriptions of Williams' lectures from 1932 to 1939, tracing more clearly the development and use of the idea of co-inherence in his thought than has been possible before. Finally, an account of the use of the word 'co-inherence' in English-speaking theology suggests that the differences that existed between Lewis and Williams, especially on the place of analogy and participation in human experience of God, might be resolved by a theology of co-inherence in the Trinity.