Ionizing radiation has benefits and risks for humankind: in cancer therapy, the cell-killing power of ionizing radiation can be used to destroy a tumour, but equally it can harm the patient through damage to healthy tissues or by causing cancer. Accurate measurement of the energy deposited in tissue —the radiation dose— is therefore needed to ensure cancer therapy is both effective and safe. Measurements of radiation dose are also required for the safety of the workforce and the general population and for other applications such as the sterilization of medical products. Harmonizing the measurement of radiation doses worldwide is therefore key to the use of ionizing radiation. There have been significant international efforts to establish and maintain a robust international measurement system, based on high-accuracy primary reference standards at metrology institutes underpinned by services at the Bureau International des Poids et Mesures (BIPM) to demonstrate the equivalence of these standards over the long term. International protocols published by the International Atomic Energy Agency (IAEA) support the clinical applications of the standards. This first article on ionizing radiation metrology explains how primary measurement standards for radiation dosimetry are realized and disseminated. A brief summary of the underpinning physics of the field is given and the main methods to construct and use primary standards for radiation dosimetry are explained.