This chapter provides an overview of breast cancer management, emphasizing both conventional therapies and advanced techniques, including Intensity-Modulated Radiation Therapy (IMRT), Volumetric Modulated Arc Therapy (VMAT), and proton beam therapy. The fundamental principles of radiobiology including repair, redistribution, repopulation, and reoxygenation are explained in detail, alongside clinical data on the α/β ratio in the linear-quadratic model and the treatment fractionation schedules. In recent decades, attention has shifted to the interaction between radiation therapy (RT) and the immune system. Numerous studies have confirmed that radiation can elicit immune responses, recruiting immune cells to the tumor microenvironment, enhancing the production of tumor-specific antigens, and inducing immunological cell death. However, this combination has not yet fully eradicated primary and metastatic lesions. The abscopal effect, a phenomenon in which low-dose radiation modifies the tumor microenvironment, may help overcome treatment resistance. This chapter explores these evolving concepts in detail, highlighting technological advancements in radiotherapy, the biological mechanisms underlying treatment resistance, and the emerging potential of combining RT with immunotherapy. By integrating physical and biological sciences with clinical innovation, the combination of RT and immunotherapy holds promise for delivering more effective and personalized care for breast cancer patients.