Total body irradiation (TBI) is becoming increasingly prominent as a major component of therapy for patients with hematological diseases requiring bone marrow transplantation. This review considers some of the special dosimetric problems associated with TBI. Basic beam dosimetry and factors affecting dose variation within the patient are discussed and compared to conventional radiotherapy. The delivery of an accurate dose to the patient requires phantom measurements to be related to the dose at the prescription point. The use of treatment planning dose calculation procedures, including inhomogeneity corrections, is described specifically for large field treatments. Finally, some questions for future research are considered.