From the beginning of life on earth, all living things have been exposed to radiation. Life started and developed in spite of, or quite possibly because of, radiation. It is disquieting to people that they coexist with radiation yet it cannot be seen, heard or felt. Radiation, when broadly defined, includes the entire spectrum of electromagnetic waves: radiowaves, microwaves, infrared, visible light, ultraviolet, and x-rays and particles. This book is concerned with radiation having energies high enough to ionize matter. Examples are x-rays, cosmic rays, and the emissions from radioactive elements. Although the term “ionizing radiation” is in this case more precise, common usage often omits “ionizing” and this is what is done here. In this book, “radiation” means “ionizing radiation”. Prior to the reactor accidents at Three Mile Island in the United States and at Chernobyl in the former Soviet Union, radiation issues were addressed primarily by specialists. Now, however, radiation and biological effects are debated by the public and political leaders. They use expressions such as: radiation dose, becquerel, sievert, cesium and γ-radiation. Because people are easily confused by this technical language, all too often they are left with the perception that all uses of radiation are dangerous. This book is written for those who want to understand radiation in order to make informed decisions about it in their lives. This field of science, founded at the turn of the century, has provided dramatic insights into physics, chemistry, biology, and medicine. The work of the early investigators provided a strong foundation from which to understand radiation phenomena. We will meet a few of them in the following pages and gain insight into their work and lives.