Radioactivity is present in all compartments of the environment and is caused by natural radiation (terrestrial - radionuclides in the Earth's crust (U-238 and Th-232 series and K-40) and cosmic - cosmogenic radionuclides (H-3, C-14, Be-7, and Na-22), as well as the anthropogenic activities (such as mining, manufacturing of fertilizers, etc.) or produced artificial radionuclides (such as Cs-137, Sr-90, I-129). The human population may be exposed to ionizing radiation emitted from various radioactive sources. Exposure to ionizing radiation from natural sources is a constant and thus inescapable factor for life on Earth. Unfortunately, man-made sources also contribute to this exposure. In addition to the risk released during the testing and use of nuclear weapons, accidents in nuclear power plants, and the nuclear fuel reprocessing industry, applications in medicine or scientific research can also contribute to an increase in radioactive contamination of the environment. Detection and distribution of radioactivity in the environment, as well as the evaluation of radioactive contamination, is a cornerstone of radiation protection. The purpose of this book is to provide information about some of the detection methods for radioactivity, as well as the potential ways for exposure to radioactivity and the risks due to exposure. This book was developed in collaboration with the authors of each chapter. Through six chapters, this book focuses on different aspects of radioactivity, from radioactive decay, content in different matrices, and detection methods, to ways of exposure to ionizing radiation.