This article reports on radiation‐induced radical reactions in gases, liquids, and solids. A brief summary of specific features of high‐energy radiation and its interaction with matter is given as a base for better understanding of reaction mechanisms initiated by radiation. The first section describes the most important features of radiolysis in a gas phase with presentation of the commonly used gas systems for inducing and studying radical reactions. Relevant examples include radical reactions connected with atmospheric and environmental chemistry, combustion, and electric discharges. The second section focuses on radiation‐induced radical reactions in water and aqueous solutions with a special emphasis on procedures that allow induction of radical reactions in a controlled manner. Relevant examples include radical reactions initiated by primary and secondary radicals from water radiolysis with a variety of compounds. Application of various organic solvents and ionic liquids offering a convenient method of generation and also some control over the nature of radical ions and excited states formed in the irradiated solvents and derived from the solutes is also presented. The last section briefly summarizes the most commonly applied solid systems and highlights radiation‐induced radical reactions in zeolites, a new matrix used recently for radiation generation of radicals and radical ions in the solid state.