Radioimmunotherapy (RIT) as a means to target radiation therapy to tumor cells or to specifically suppress host immunity specifically in the setting of allogeneic transplantation is a promising new strategy in the armory of today's oncologist. Different approaches of RIT such as injection of a stable radioimmunoconjugate or the use of pretargeting are available. The choice of the radionuclide used for RIT depends on its radiation characteristics with respect to the malignancy or cells targeted. beta-Emitters with their lower energy and longer path length are more suitable for targeting bulky, solid tumors, whereas alpha-emitters with their high linear energy transfer and short path length are better suited to target cells or tumors of the hematologic system. Encouraging results have been obtained using these approaches treating patients with hematologic malignancies. While the results in solid tumors are somewhat less favorable, new strategies for patients with minimal residual disease (MRD), using adjuvant and locoregional treatment, are currently being investigated. In this report, we outline basic principles of RIT, give an overview of available radioimmunoconjugates and their clinical applications with special emphasis on their use in hematologic malignancies, including use in conditioning regimens for stem cell transplantation (SCT).