T cells contain a variety of lineages, each of which is formed by passing through a number of strictly regulated steps. In this article we aim to clarify essential factors and critical developmental steps during T cell development. First, we will describe the process of T cell development that occurs in vivo. Then, by presenting recent approaches in synthetic biology, we will show that, in the murine case, a feeder-free culture system using a combination of cytokines and Notch ligand is sufficient to support T cell development starting from multipotent hematopoietic progenitors to the TCR-expressing CD4+CD8+ stage. Finally, in the human case, we will show that mature functional CD8+ killer T cells can be produced in vitro starting as early as from the ES/iPS cell stage using feeder cells. These studies may help clarify “minimal requirements” for T cell development.