Nuclear medicine is a medical specialty that uses radiopharmaceuticals for diagnostic imaging or therapy. This radiopharmaceutical compound consists of a radionuclide optionally coupled to a vector which binds specifically to biological targets of interest. This product has been developed with the aim of being as selective as possible at the level of the tumor, while preserving healthy tissue as much as possible. For therapeutic applications, the β− or α radiation emitted by the radionuclide is used to kill the targeted cells, so-called radionuclide therapy (RNT). For several decades, radionuclide therapy has mostly been limited to the use of iodine-131 in various thyroid diseases. In recent years, advances in the understanding of molecular mechanisms underlying cancer biology, followed by the appearance of new vectors (antibodies, peptides, small molecules) and the availability of new radionuclides, have pushed radionuclide therapy toward personalized treatment, with better therapeutic efficacy and radiation safety. In particular, the tumor microenvironment now appears as a potential target for drug delivery, including radiopharmaceuticals, alongside tumor cell membrane receptors or antigens. Several therapeutic receptor-specific radiopharmaceuticals have demonstrated their clinical usefulness in a variety of tumor types and have been or are on the verge to be granted authorization. Following the clinical success of these latter, research in that domain is very active, and the clinical pipeline appears particularly promising. Receptor-specific radionuclide therapy clearly embodies Paul Ehrlich’s concept of “magic bullet.”