Radiation therapy (RT) is an important component of the management of lymphoma patients. Most lymphomas are metabolically active and accumulate 18F-fluorodeoxyglucose (FDG). Positron emission tomography with computer tomography (PET/CT) imaging using FDG is used routinely in staging and treatment evaluation. FDG-PET/CT imaging is now also used routinely for contouring the target for RT, and has been shown to change the irradiated volume significantly compared with CT imaging alone. Modern advanced imaging techniques with image fusion and motion management in combination with modern highly conformal RT techniques have increased the precision of RT, and have made it possible to reduce dramatically the risks of long-term side effects of treatment while maintaining the high cure rates for these diseases.