Bone marrow transplantation usually entails the ablation of the recipient's immune system. The recovery of immunity after marrow transplantation is a complex process dependent on a number of pre- and posttransplantation factors. Many of the basic concepts of immune reconstitution after transplantation have been elucidated over the past decade. Recent developments focus on attempts to manipulate immune recovery in the posttransplantation period. Soluble mediators of both B-cell and T-cell number and function have been tested in both preclinical and clinical transplant models. The direct administration of T cells after transplantation can be used to augment the immune response to infection, lymphoproliferative disease, and malignancy.