Radiotheranostics integrates molecular imaging and targeted radionuclide therapy to provide personalized cancer treatment. By using radiolabeled agents that bind tumor-specific targets, clinicians can both visualize and treat malignancies, guided by the principle of “treat what you see.” Diagnostic tools such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) confirm target presence, while therapeutic radioisotopes deliver cytotoxic radiation to tumor cells, enabling real-time monitoring and individualized treatment. These agents pair a targeting molecule—such as a peptide, small molecule, or antibody—with either a diagnostic or therapeutic radioisotope. Beta and alpha emitters offer different radiation profiles, suited for specific clinical needs. This strategy has already improved outcomes in diseases like neuroendocrine tumors and prostate cancer, and new applications are rapidly emerging. Radiotheranostics provides a systemic alternative to external beam radiotherapy, allowing for personalized dosimetry to enhance efficacy and minimize side effects. The field has evolved significantly from early radioiodine therapy, driven by advances in radiochemistry and imaging. While treatments are generally safe, potential toxicities—such as bone marrow suppression or glandular damage—require careful management. As radiotheranostics advances toward broader clinical use, overcoming challenges in isotope supply, infrastructure, and workforce training will be critical. Its integration into multidisciplinary oncology teams promises to make it a cornerstone of precision cancer care.