Background Radiological examination is a common diagnostic practice in modern medicine, they are not uncommonly performed during pregnancy or childhood. The potential biological effects of radiation to both the developing fetus and children are not always clear and remained controversial over many years. Physicians who care for these patients always find it difficult to evaluate the risk, and have misconceptions regarding the use of ionized radiation in pregnancy and children, which may delay the management process. Objective This study has reviewed all recent published observational studies, and analyse any possible association of prenatal or postnatal X-ray exposure from diagnostic imaging and childhood cancer risk. Methods Eligible epidemiological studies published between January 2000 and June 2013 were reviewed. These studies were found through electronic searches using Medline, PubMed, Embase, and Cochrane Database. Predetermined inclusion and exclusion criteria were applied to the identified articles Results Twenty-five articles with fourteen million participants were recruited. 17 out of 25 were case-control studies and 8 were cohort studies. All studies tried to prove an association between X-ray or CT scan exposure, and cancer of the haematopoietic system, brain and soft tissue regions. Results were summarized separately for their study methods, mode of radiation exposure and for each cancer outcome. The quality of the articles was accessed with the Newcastle-Ottawa scale. The overall OR estimate from case-control studies showed postnatal X-ray exposure positively and significantly associated with leukaemia risk (OR 1.21; 95% CI: 1.10-1.32; I2 = 3%). Cancer risk other than leukaemia are lacking in case-control studies. Recent cohort studies also showed a small but significant increase risk of leukaemia and brain tumour from childhood CT scan exposure. Conclusion This analysis had shown a small but significant increase cancer risk from X-ray or CT scans exposure in postnatal period. Varies measures should be used to minimize the radiation dose in children during radiation exposure. As long as the radiological imaging is clinically indicated and performed using appropriate scanning protocol, the benefits of radiological imaging should far outweigh the small radiation risk. / published_or_final_version / Public Health / Master / Master of Public Health