The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) assesses the levels and effects of exposure to ionizing radiation. UNSCEAR's scientific findings underpin radiation risk evaluation and international protection standards. The scientific annexes provide the supporting scientific rationale for UNSCEAR's 2017 report to the General Assembly. Annex A provides the principles and criteria for ensuring the quality of the Committee's reviews of epidemiological studies of radiation exposures, with a focus on the assessment of strengths and limitations of such studies. Annex B evaluates epidemiological studies analysing cancer risk on the basis of individual doses due to exposure to low dose rates from environmental sources, including key limitations of such studies (e.g. statistical power, dosimetric uncertainty and confounding).