The phenomenon of trial by ordeal has been observed across many cultures and eras, taking a number of different forms. Frequently described as 'irrational', the ordeal is a physical test, usually incorporating a supernatural element, which is used to determine the innocence or guilt of an accused person. This thesis looks at trial by ordeal alongside other related modes of divination and irrational proof in the ancient Near East, medieval and early modern Europe and twentieth century Africa to produce a conceptually coherent account of trial by ordeal and place it in a theoretical framework. In addition to this, the thesis considers the workings of the ordeal, looking at issues of correctness versus closure, legitimate and illegitimate uses of magic as well as the possibility, form and effect of human manipulation of the process. It concludes by offering a more nuanced definition of trial by ordeal set in the context of an understanding of its social functions and operation.