Diagrams represent a critical chapter within the history of the development of techniques of visualization, whether in philosophy, science, mathematics or the arts. Having been disparaged through much of the twentieth century, diagrams today enjoy a renaissance due, in large measure, to the revolution represented by computers, but also on account of reevaluations of what constitutes an image. Deployed in almost every area of human endeavor, diagrams both enable and impose the ordering of knowledge and perception. In no period of western art was this truer than in the Middle Ages, much of whose art can indeed be described as diagrammatic, for reasons rooted in theology and cosmology. Working from medieval examples—in particular, those related to representations of the Cross, itself a fundamentally diagrammatic image—but embracing modern discourses on diagrams from semiotics (Peirce) to post-structuralism (Deleuze & Guattari), the lecture considers the implications of diagrammatic modes of thought and representation on such age-old philosophical questions as what is an image and what relationship do images have to the truth.