Metaiodobenzylguanidine (mIBG), developed 30 years ago, is taken up by tumours expressing the noradrenaline transporter. Radiolabelled with I-123 or I-131, mIBG has become a gold standard for diagnostic imaging of pediatric and adult neuroendocrine cancer. Within a few years of its clinical introduction, I-131 mIBG was found to be an effective palliative treatment with minimal toxicity that in some cases could produce a complete response. The importance of internal dosimetry for I-131 mIBG therapy has been demonstrated by a number of studies showing that absorbed doses delivered to tumours and organs-at-risk from standard and weight-based activities can vary by an order of magnitude. However, significant correlations between the whole-body absorbed dose and myelotoxicity have been demonstrated and studies based on this relationship have enabled treatments to be tailored to the individual. Ongoing developments include patient-specific treatment planning based on tumour dosimetry and cocktails of radionuclides and radiosensitisers.