Considerable progress has been achieved in the identification and analysis of regulatory molecules that orchestrate B and T cell fate determination. IL-7 and Notch signaling appear to represent analogous pathways that direct B and T cell fates, respectively. For the B lineage, it is possible to assemble the various signaling components and transcription factors into a coherent gene regulatory network. At present, it is not possible to assemble an equivalently precise network for the T lineage as fewer components have been analyzed and their connectivity remains to be more fully explored. However, the ability to manipulate T cell development in a simple culture system using Notch ligand-expressing stromal cells is greatly facilitating analysis of this pathway.