This article systematically examines the scientific nature of radioactive substances, the principal types of ionizing radiation, their biological effects, areas of application, and the fundamental principles of radiation protection. Radioactivity is defined as the spontaneous transformation of unstable atomic nuclei, and key concepts such as radionuclides, radioactive decay, activity, and half-life are explained within a scientific framework. Natural and artificial radioactivity are compared, while the characteristics of alpha, beta, gamma, and neutron radiation are discussed in detail. The article analyzes the mechanisms through which ionizing radiation affects living organisms, including DNA damage, as well as deterministic and stochastic biological effects. The practical applications of radioactive substances in medicine, industry, nuclear energy, and agriculture are evaluated alongside the associated benefits and potential risks. Furthermore, the fundamental principles of radiation safety—justification, optimization (ALARA), and dose limitation—are examined. Particular attention is given to the principles of time, distance, and shielding, methods of protection against internal and external exposure, dosimetric monitoring, and the management of radioactive waste. The study concludes that the safe utilization of radioactive substances can only be ensured through the integration of scientific knowledge, regulatory frameworks, and professional oversight. Special emphasis is placed on the importance of fostering a strong safety culture and implementing continuous monitoring systems. Such an approach contributes significantly to the effective protection of human health and the environment.