The mature thymic epithelium is complex, with two major compartments - the cortex and the medulla - each containing several functionally distinct epithelial-cell types. There is considerable debate as to the embryonic origins of these different thymic epithelial-cell subpopulations. The textbook view is a dual origin, with cortical thymic epithelium arising from the ectoderm and medullary thymic epithelium originating in the endoderm. However, the literature has been divided on this issue since it was first considered. In this review, we discuss recent embryological, functional, genetic and molecular data that collectively support a new model of thymus organogenesis and patterning.