The first unequivocally successful bone marrow cell transplantation in humans was recorded in 1968 by the Universitv of Minnesota team of Robert A.Good.This achievement was a direct extension of mouse models of acquired immunologic tolerance that were established 15 years earlier.In contrast,organ(i.e.kidney)transplantation was accomplished precociously in humans(in 1959)before demonstrating its feasibility in any experimental model and in the absence of a defensible immunologic rationale.Because of the striking differences between the outcomes with the 2 kinds of procedure,the mechanisms of organ engraftment were long thought to differ from the leukocyte ehimerism-associated ones of bone marrow transplantation.This and other concepts of alloengraftment and acquired tolerance have changed over time.Current concepts and their clinical implications can be understood and discussed best from the perspective provided by the life and times of Bob Good.