Since its beginning, more than 16,000 patients have been enrolled in the multicentric randomized trials of the German Hodgkin Study Group (GHSG) for adult patients. Within 6 study generations, the treatment of Hodgkin lymphoma has been developed stepwise by using the results of the completed protocols. Now the sixth generation is active. According to the role of radiotherapy, the study group successfully evaluated different dose–effect relationships and could also prove the efficacy of involved-field radiotherapy in early stages in combination with effective chemotherapy. Currently, a radiation dose of 20 Gy to the involved field after a mildly aggressive chemotherapy (2 cycles of adriamycin, bleomycin, vinblastine, and dacarbazine [ABVD]) should be the standard for early-favorable stages according to the GHSG classification. In early-unfavorable or intermediate-risk stages, involved-field radiation therapy with 30 Gy is sufficient; the optimal chemotherapy has to be fixed. For the advanced stages, the question of radiotherapy is still unclear. Preliminary results of the GHSG and others show that additive radiotherapy after intensive chemotherapy might be useful for elective subgroups of patients. The extensive radiotherapy quality assurance program, performed by the GHSG and its radiotherapy reference center, has proven to be successful and necessary to ensure that, with reduced radiation doses and reduced radiation volumes, precise radiotherapy, as defined by the protocol, will be performed by the participating radiotherapy departments.