Some terrestrial microorganisms can survive in extremely severe environments, such as high temperature, high salinity, strongly acidic, or strongly alkaline conditions. Elucidation of the adaptive mechanisms of these extremophiles to extreme environments will provide meaningful insights into the evolution and diversity of organisms. Deinococcus radiodurans, a representative radioresistant bacterium, are extremely resistant to ionizing radiation and other DNA-damaging factors. The D37 of D. radiodurans is 7 kGy, for example, while that of Escherichia coli is only 40 Gy. The tolerance of D. radiodurans to radiation is due to high DNA repair rates, high antioxidant activity, specialized cell structures, and uncommon intracellular environments; this bacterium is considered a model organism in the study of DNA damage and repair. This paper summarizes the recent research on the diversity of radioresistant bacteria, including their environments, factors that influence resistance to DNA damage, mechanisms of DNA damage and repair, and central repair genes, which are associated with the repair of double-stranded DNA breaks in Deinococcus radiodurans. The potential applications of this knowledge to life sciences, agriculture, contaminative environmental biorepair, and medicine are also discussed.