This book aims to explain radiation from a somewhat different aspect than its traditional image as something that is scary, dangerous, hazardous, and so on, to produce the correct understanding that radiation is carrying energy, and to convince readers that radiation is not "scary" but controllable and useful. As for radiation itself, many introductions or textbooks have been published, as in radiochemistry, radiobiology, and radiology. In most of them, the biological effects of radiation exposure are the main subjects, which often enhance the feeling that radiation is dangerous, and the effects produced by lower-dose exposure that are difficult to see are hardly discussed. The present volume mainly focuses on how radiation carries energy, how energy is absorbed in substances as absorbed doses (Gy) or dose equivalents (Sv), how damages or risks appear with the absorbed dose and why the effects of the exposure appear quite differently, depending on properties of the substancesthat were exposed.