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 third of four volumes of scientific annexes provide the supporting scientific deliberations for the UNSCEAR 2020/2021 report to the General Assembly. Annex C 'Biological mechanisms relevant for the inference of cancer risks from low dose and low-dose rate radiation' synthesizes the current knowledge on biological mechanisms of radiation actions at low doses and low-dose-rates, and assesses the implications for understanding the processes of cancer development after exposure to ionizing radiation, and for dose-response relationships of radiation-induced cancers