Ionizing radiation (RT) delivered to solid tumors is well known to induce immunological changes within the local tumor microenvironment as well as elicit systemic responses that promote antitumor immunity. These immunomodulatory effects of RT are produced by multiple processes that include increased tumor-associated antigen release, enhanced immunogenic cancer cell death, and upregulation of proinflammatory cytokines, which ultimately promote T cell priming and activation. RT can also modify the tumor microenvironment to facilitate immune effector cell recruitment, maturation, and activation. Given these properties, RT may augment the effect of modern cancer immunotherapies. This review outlines the fundamental immunogenic properties of RT and presents an overview of the clinical experience thus far that support the combination of RT with immunotherapy in cancer treatments.