The bulk of the corneal stroma is comprised of a layered network of fibrillar collagen. Determining the architecture of this unique structure may help us to better understand the cornea's biomechanical and optical function. The analysis of diffraction patterns obtained when X-rays are passed through the regularly arranged collagen molecules and fibrils of the stromal matrix yields quantitative data on fibrillar organisation, including the orientation and distribution of collagen lamellae within the corneal plane. In recent years, by exploiting the radiation from powerful synchrotron sources, techniques have been developed to enable the mapping of collagen fibril, and therefore lamellar, directions across whole corneas. This article aims to summarise the use of X-ray diffraction to map the orientation and distribution of collagen in the corneal stroma. The implications of the knowledge gained so far are discussed in relation to the optical and biomechanical properties of the cornea, and their alteration due to disease and surgical intervention.