Bone marrow transplantation has become well established in the treatment of malignant and nonmalignant disorders. Steady progress continues in improving survival after transplantation, but many problems remain to be solved. Relapsed disease is a major limitation, and new, more intensive preparative regimens are being evaluated in an effort to improve efficacy and reduce toxicity. The availability of unrelated donors has been improved with continued expansion of the National Marrow Donor Program. Additional alternative sources of marrow actively being investigated include partially matched family members and peripheral blood stem cells. Improved marrow purging techniques are also being studied to allow expanded use of autologous marrow for various malignancies. The use of cytokines to minimize regimen toxicity and as therapeutic agents is also promising. The late effects of bone marrow transplantation, including relapse, chronic graft-versus-host disease, secondary malignancies, and effects on growth and development, are important considerations in the design of new therapeutic strategies.