This is an exciting time for targeted molecular radiotherapy. Recent advances in the field of theranostics are enabling the implementation of innovative approaches for the treatment of a plethora of systemic malignancies. Theranostics introduced a paradigm shift in medicine by offering the advantage of combining specific targeted therapy based on specific targeted diagnostic tests. In the era of precision medicine, it is time to revive the debate on standardized versus personalized or patient-individualized dosimetry. It is gratifying to see in perspective the progress made in internal dosimetry, from conventional fixed amounts administered to all patients to patient-specific dosimetry based on actual patient data and molecular imaging tools, advanced computational modeling of the human body, and sophisticated Monte Carlo-based calculation techniques enabling accurate patient-specific estimates of the delivered radiation dose. It is becoming widely accepted that personalized dosimetry enables a more rigorous approach for treatment planning aiming at improved understanding of the dose–response relationship.