According to the available data, the effect of high doses of ionizing radiation on the human central nervous system (CNS) takes form of cognitive dysfunction and increased risk of development of malignant neoplasms. At the same time, there is a growing concern about the possible effects of low, moderate doses of ionizing radiation and chronic irradiation, on cognitive functions, as well as their potential long-term consequences manifesting as neurodegenerative diseases. There is both epidemiological and experimental evidence confirming that low and moderate doses of ionizing radiation affect cognitive abilities. The underlying mechanisms include disruption of normal neurogenesis in the hippocampus, development of long-term sustained neuroinflammation, disorders of synaptic plasticity, energy metabolism, and oxidative status. On the part of CNS, the body is most sensitive to radiation during the period of active formation of the brain. Irradiated at that time, people may suffer consequences thereof for several months and years, or have them manifesting only much later, in old age. Improvement of radiation safety and development of means and ways of prevention and treatment of radiation-induced CNS disorders require further research efforts aimed at establishing causal relationships between chronic exposure to radiation and low-dose irradiation and their adverse effects on the part of CNS in the long term post-exposure.