High-dose chemoradiotherapy followed by autologous or allogeneic marrow rescue has revolutionized oncology and become the new standard of care for a variety of malignant diseases previously treated with conventional dose chemotherapy. Peripheral blood stem cells (PBSC) promise to provide a similar revolution for the source of hematopoietic stem cells. In this chapter, the term PBSC is used to define a heterogeneous population of committed and uncommitted hematopoietic precursor cells in the peripheral blood that in toto are capable of short- and long-term engraftment. The relative ease of collection and the markedly shorter recovery time for peripheral counts after marrow ablative therapy suggest that PBSC will supplant marrow for both allogeneic and autologous transplantation in the next 5 years.