Exposure of human body to neutrons occurs in radiotherapy using high-energy radiations. This review summarizes knowledges related to biological effects of neutrons, including those obtained in recent projects in Japan and Europe. A study of Japanese atomic bomb survivors with recently revised dosimetry indicated very high relative biological effectiveness (RBE) of 25–80 (as point estimates) regarding cancer risk. Animal studies indicate RBE of 2–100 or even higher regarding cancer induction, which seem to have a peak around ～1 MeV. Evidence suggests that these values depend on the age and sex. Reported RBE regarding the effects on the lens of the eye is in a similar range and sometimes very high. Regarding other tissue reactions, reported RBE values range from 2–10. Experiments at the cellular level have reported RBE of 1–5 regarding cell killing, 2–20 regarding induction of mutations (with a peak at ～1 MeV), and ～1 regarding induction of DNA double strand breaks. A simulation study predicted that the RBE of induction of complex DNA breaks peaks at ～1 MeV with a value of ～17. The complex breaks produced are likely to be far less in amount than simple DNA breaks, leading to a subtle increase in the yield of total DNA breaks; however, these complex damages may be very efficient in inducing mutations and cancer. Thus, the combination of the yield of complex DNA damage and its efficacy in inducing cancer is considered to underlie the high RBE of neutrons regarding cancer risk.