The purpose of the book is to make the subject of radiation understandable to the general public, without scientific compromises, so that responsible citizens can assess potential risks to their health, quality of life, and environment. Starting with a retrospective of our perceptions, we enter the microcosm of matter, the scales of the atom and the nucleus, we learn their structure and the elementary particles that constitute them, and we reveal nuclear power. The nature, properties, and effects of radiation are presented in a simple and understandable way, as well as the characteristics of the natural environment of radiation in which humans have always lived and developed. The principles of nuclear reactors' operation, their safety, the controversial issue of nuclear waste management, the applications of nuclear technology in the fields of health and production, and finally, the history, causes, and consequences of the Chernobyl accident, with an emphasis on its effects in Greece, are described.