Hot-atom chemistry is a unique field of chemistry dealing with highly excited chemical species resulting from nuclear reactions or radioactive decay processes. Modern hot-atom chemistry includes a broad range of disciplines such as fundamental studies from physical chemistry of gas-phase energetic reactions to inorganic solid-state chemistry, as well as recent practical applications in life sciences and energy-related research. In spite of the importance of hot-atom chemistry and its appli­ cations, its relevance to the other fields of chemistry and related disciplines has attracted little attention and only books and review articles for dedicated hot-atom chemists have been published to date. In this volume, we illustrate the essential aspects of modern hot-atom chemistry for non-specialists, with considerable emphasis on its applications in the related fields. We sincerely hope that this volume can promote mutual understanding and collaboration between hot-atom chemists and researchers in other disciplines. After a brief introduction (Chap. 1) the 2nd chapter gives the non-specialist an idea of experimental techniques commonly used for the production and analysis of hot chemical species. In Chap. 3, we have explained the concepts of hot-atom reac­ tions in gas, liquid and solid phases with typical examples rather than a comprehensive review of the literature. In view of the current state of accomplishment, the greater part of this chapter is concerned with gas phase studies. Regarding the solid-phase hot­ atom chemistry, we have confined ourselves only to introducing new concepts and discussing modern aspects.