A number of therapeutic agents in nuclear medicine are currently attracting considerable interest, including several for the treatment of hematologic and nonhematologic malignancies. A knowledge of the radiation dose received by different organs in the body is essential to the optimization of the therapy for each patient; one wants to maximize the dose to the malignant tissue while minimizing the dose to critical healthy tissues and avoiding any toxic response therein. In this paper, current methods for calculating radiation doses will be discussed and evaluated. In almost all nuclear medicine therapy, and particularly in this application, dose to the active marrow is of paramount concern. Specific focus on current bone marrow dose models and their ability to predict observed marrow toxicity in patient populations to date will be discussed. The paper will focus on current and possible future dosimetry practice in therapeutic nuclear medicine, particularly as regards the treatment of hematologic malignancies.