Radiopharmaceuticals are radiolabeled compounds used for diagnostic or therapeutic purposes in humans. These compounds consist of a molecular or biological vector that determines their biodistribution and kinetics within the body and a radioactive isotope (radionuclide) that emits radiation. The specific application of radiopharmaceuticals, whether for diagnosis or therapy, depends on the type of radiation emitted by the radionuclide. Gamma rays, characterized by their high penetration power and low linear energy transfer (LET), are primarily used for imaging (diagnosis). In contrast, alpha- and beta-radiation, which have low penetration power and high LET, are employed for therapeutic purposes. In this chapter, we explore the properties of radiation and their corresponding biological effects. We discuss how these properties relate to the specific applications of radiopharmaceuticals, highlighting the suitability of particular radionuclides for diagnosis or therapy. Additionally, we provide an overview of nuclear imaging technologies widely used in diagnostic nuclear medicine. We also outline the history of radiopharmaceutical sciences, presenting a comprehensive timeline of key discoveries in the field. Finally, we offer examples of the most significant and promising radiopharmaceuticals currently in clinical use or undergoing clinical evaluation.