Increasingly, hemibody and total body irradiation are used to treat systemic and disseminated diseases. Certain dosimetry considerations are common to conventional treatments at short source‐to‐axis distances and large field treatments at extended treatment distances. These dosimetry considerations include: irradiation geometry, dose rates or outputs in the treatment plane, radiation intensity uniformity, confirmation of the dose ratio parameters such as percentage depth doses, tissue air ratios, tissue maximum ratios, and tissue phantom ratios, the surface doses and the doses in the region of build‐up to maximum ionization, the exit doses, the dose prescription point or points, dose uniformity in the patient, and dose confirmation and verification. Some radiotherapists are reluctant to treat patients using large fields at extended treatment distances because they are unfamiliar with the dosimetry of such treatments. This article reviews the dosimetry and radiobiology of large field treatments at the extended treatment distances required by hemibody or total body irradiation, as performed with conventional single source teletherapy units.