Dr Shortman is distinguished for his work on the differentiation pathways of lymphoid cells, in particular those leading to T lymphocytes and dendritic cells. He was a pioneer in the development of biophysical procedures for the purification of distinct lymphoid cell subsets and was the first investigator to succeed in isolating the earliest thyrnic lymphoid progenitor. A quantitative approach has been the hallmark of his style and he has given us a clear picture of the kinetics of the birth, proliferation, emigration, and death of cells in the thymus.