<p>Ionizing radiation-induced drug&#xa0;release is a combined chemoradiation therapy, which aims to reduce the systemic toxicity of chemotherapeutics. Radiation is used for both radiotherapy and to trigger the release of a chemotherapeutic. To understand radiation-induced drug activation and to design new radiation-sensitive chemotherapeutics, it is important to become familiar with the underlying reaction mechanisms. Here, we provide an overview of the crucial process of water radiolysis induced by ionizing radiation and the mechanisms of reactive species generation. We also discuss the reactivity of these species with cellular components and chemical functional groups, to give insight into selective drug activation in complex cellular environments. Finally, we discuss recent progress on radiation-induced drug release focusing on the reaction of water radiolysis products with drug caging groups and the yield of released drugs. We aim to bridge the gap between basic chemical processes in water radiolysis and their relevance for drug release and provide suggestions on the design of radiation-sensitive prodrugs or nanocarriers.</p><p></p>