We have considered it to be a demanding assignment to provide a complete exposition dealing with the nature of radiation, its effects, and protection against it to workers in health-related activities. Radiation (and more precisely ionizing radiation) is emitted by X-ray machines, nuclear reactors, and nuclear weapons, but also comes from natural sources to which we are all exposed. It would have been easier to deal with this subject area with the terminology and mathematics employed by specialists. However, although most of the potential readers probably have obtained further pertinent knowledge, we assume no more than a high school education in science and mathematics and the challenge was to provide maximum information within this constraint. This book contains five sections: (A) Radiation Physics, (B) Radiological Physics, (C) Radiation Biology, (D) Radiation Effects on Human Populations, and (E) Radiation Protection. Each section is preceded by a synopsis covering its essential features. It provides sufficient information to enable readers to obtain a general under standing of the subject of the section and an adequate background for comprehension of other sections. The more detailed presentation in the bulk of each section is followed by appendixes that generally contain more advanced topics. This scheme necessarily involves some repetition but permits a more flexible approach for readers who are especially interested in the contents of particular sections.