The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) established by the General Assembly in 1955 assesses the levels and effects of exposure to ionizing radiation on human health and the environment. This is the first of four volumes of scientific annexes that presents the supporting scientific deliberations for the UNSCEAR 2020/2021 report to the United Nations General Assembly. Annex A “Medical exposure to ionizing radiation” presents a comprehensive global estimate of the frequencies and doses from medical exposure, their distribution and trends in medical categories in the 2009–2018 period. The annex considers four general categories of medical practice using ionizing radiation: (a) diagnostic radiology, including dental radiology and computed tomography, (b) interventional radiology (image-guided interventional procedures), (c) nuclear medicine and (d) radiation therapy.