The advent and success of immune checkpoint inhibitors (ICIs) in cancer treatment has broadened the spectrum of tumours that might be considered "immunogenic" and susceptible to immunotherapeutic (IT) intervention. Not all cancer types are sensitive, and not all patients with any given type respond. Combination treatment of ICIs with an established cytotoxic modality such as radiation therapy (RT) is a logical step towards improvement. For one, RT alone has been shown to be genuinely immunomodulatory and secondly pre-clinical data generally support combined ICI-RT approaches. This new integrated therapy for cancer treatment holds much promise, although there is still a lot to be learned about how best to schedule the treatments, manage the toxicities and determine what biomarkers might predict response, as well as many other issues. This review examines how RT alters the immune rheostat and how it might best be positioned to fully exploit IT.