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 last of four volumes of scientific annexes provide the supporting scientific deliberations for the UNSCEAR 2020/2021 report to the General Assembly. Annex D 'Evaluation of occupational exposure to ionizing radiation' presents the estimated average annual effective and collective doses to workers for each major work sector and sub-sector involving the use of ionizing radiation; estimated worldwide level of occupational exposure for different sectors involving exposure to natural sources and to human-made sources of radiation; and analysis of temporal trends in occupational exposure. It is based on data provided by United Nations Member States and international organizations, as well as peer reviewed literature and national reports on radiation exposure to workers