Although experimental as well as epidemiological studies have revealed the health effects of ionizing radiation, most of our knowledge is for high doses of radiation, while little is known for low doses. For practical purposes, we estimate the risk of low dose radiation by extrapolating the effects at high doses to low doses in a linear relationship. However, several lines of evidence have accumulated in recent years that suggest this linear extrapolation is not necessarily correct and needs further scientific evaluation. Today, many scientists in the field are striving to understand the biological responses to low dose radiation. This work will provide new and perhaps convincing data which are necessary for risk estimation of low dose radiation. Here, I overview the background of the issue.