Unstable isotopes and their capacity to emit ionizing radiation have been employed in clinical practice not only for diagnostic, but also for therapeutic purposes, with significant contribution in several fields of medicine and primarily in the management of oncologic patients. Their efficacy is associated with their ability to provide the targeted delivery of ionizing radiation for a determined duration. These compounds can be used for curative or palliative treatment, as well as for a diagnostic-therapeutic (theranostic) approach. This review summarises the most recent trends in radionuclide treatment for several malignancies, including prostate cancer, neuroendocrine tumours, and hematological and thyroid malignancies, in which radionuclide-based therapies have been employed with high effectiveness.