MicroRNAs (miRNAs) are small non-protein coding RNA that play an important role in gene regulation. These RNA molecules function as post-transcriptional regulators. miRNAs bind to complementary sequences on target messenger RNA transcripts, usually resulting in translational repression or target mRNA degradation and gene silencing. miRNA are abundantly present in all human cells, target approximately 60% of all genes, and are able to repress hundreds of targets each. Since their discovery in 1993 miRNA are emerging as important modulators in cellular pathways such as growth and proliferation, apoptosis, carcinogenesis, timing of cell-fate decision, and metabolic pathways. A large number of studies have examined the general and specific effects of miRNAs perturbation in radiation-exposed cells. These studies include expression profiling of miRNA, functional analysis, the role of specific miRNAs in tumor radiosensitivity, and targeting miRNA for improved cancer radiotherapy. Other studies have explored the involvement of miRNA in radiobiological phenomenon like bystander effect. Emerging evidence is establishing that miRNA are involved in regulating radiation-induced cellular processes, can be exploited to improve cancer radiation therapy, and could serve as biomarkers of human radiation exposure. J. Cell. Biochem. 115: 436–449, 2014. © 2013 Wiley Periodicals, Inc.