Allogeneic hematopoietic stem cell transplantation (HCT) is a potentially curative treatment modality for patients with a variety of hematological malignancies and non-malignant diseases. Several drugs are used in the conditioning phase of HCT, depending on the clinical features of the disease. Radiotherapy (RT) may be effectively integrated in the conditioning regimen of HCT, in combination with systemic agents. This comprehensive chapter describes rational, clinical indications, and practical aspects of the different radiotherapy (RT) techniques and schedules that can be used in this peculiar setting. Along with a thorough description of the different RT planning and delivery phases, we present in detail the potential side effects of these highly peculiar treatments and discuss the actual state-of-art schedules and future perspectives.