Since the Fukushima nuclear plant accident following the great East Japan earthquake on March 11, 2011, we have been warned to be careful about possible radiation exposure almost every day in newspapers and on TV. Radioactive iodine (¹³¹I) and cesium (¹³⁴Cs, ¹³⁷Cs) produced by nuclear reactions were released into the air during and after the accident, and have been scattered by the winds in Tohoku and in the Kanto district. Even today, 2 years after the accident, there is great public concern about possible pollution of foodstuffs and fishery products with radioactive cesium, not only in Japan, but also in other countries. On the other hand, decontamination work has been proceeding, including removal of contaminated soil near the accident site. Since the accident, many media reports have continued to tell us only that current dose levels of radiation are not dangerous to human health. But, many people are not satisfied with such vague statements, and want to understand the situation in more detail. So, it is important to provide basic education about the effects of radiation to the general public. I am a professor of the Department of Radiation Biosciences at Tokyo University of Science, and so I am very familiar with radiation and its dangers. So, in my lecture today, we would like to explain the effects of radiation and put the present situation into perspective, so that people will better understand the risks, and not be unnecessarily afraid.