The thymus is a primary lymphoid organ composed of a three-dimensional (3D) epithelial networkEpithelial network that provides a specialized microenvironment for the phenotypical and functional maturation of lymphoid progenitors. The specification of the pharyngeal endodermPharyngeal endoderm to thymus fate occurs during the early stages of thymic organogenesisOrganogenesis, independent of the expression of the transcription factorTranscription factor Foxn1Foxn1. However, Foxn1 governs the later organogenesisOrganogenesis of thymus together with the colonizing lymphoid cells. In the present chapter, we will review recent evidence on the topic covered in our original chapter (Muñoz and Zapata 2019). It described the early development of thymus and its resemblance to the development of endoderm-derived epithelial organs based on tubulogenesis and branching morphogenesis as well as the molecules known to be involved in these processes.