Radiotherapy for cancer has been known to affect the responses of immune cells, especially those of CD8+ T cells that play a pivotal role in anti-tumor immunity. Clinical success of immune checkpoint inhibitors led to an increasing interest into the ability of radiation to modulate CD8+ T cell responses. Recent studies that carefully analyzed the CD8+ T cell responses following radiotherapy suggest the beneficial roles of radiotherapy on anti-tumor immunity. In addition, numerous clinical trials that evaluate the efficacy of combining radiotherapy with immune checkpoint inhibitors are undergoing. In this review, we summarize the current status of knowledge regarding the changes of CD8+ T cells following radiotherapy from various preclinical and clinical studies. Furthermore, key biologic mechanisms that underlie such modulation, including both direct and indirect effects, are described. Lastly, we discuss the current evidence and essential considerations for harnessing radiotherapy as a combination partner for immune checkpoint inhibitors.