<p>Radiopharmaceuticals, containing radionuclides, are crucial for medical diagnosis and treatment. Their development has progressed through four distinct stages: the initial exploratory stage; the development of nuclear medicine imaging technologies; the rise of radiolabeled targeted agents; and the advent of theranostics, ushering in a new era. Early research had limited therapeutic efficacy success. The development of nuclear imaging technologies improved diagnostics. The emergence of radioconjugates enabled precise and targeted therapy for tumors. The theranostic approach, integrating diagnosis and treatment, has facilitated the advancement of personalized medicine. This review summarizes the key events, radionuclide applications, advantages, and limitations across these four phases, offering a perspective on future directions in the field.</p>