A huge earthquake struck the northeast coast of the main island of Japan on 11 March 2011 triggering an extremely large tsunami to hit the area. The earthquake and tsunami caused serious damage to the Fukushima nuclear power plants (NPPs) of Tokyo Electric Power Company (TEPCO), resulting in large amounts of radioactive materials being released into the environment. The major nuclides released were 131I, 134Cs and 137Cs. The deposition of these radioactive materials on land resulted in a high ambient dose of radiation around the NPPs, especially within a 20-km radius. Dose assessments based on behavior survey and ambient dose rates revealed that external doses to most residents were lower than 5 mSv, with the maximum dose being 25 mSv. It was fortunate that no workers from the NPPs required treatment from the viewpoint of deterministic effects of radiation. However, a lack of exact knowledge of radiation and its effects prevented the system for medical care and transportation of contaminated personnel from functioning. After the accident, demands or requests for training courses have been increasing. We have learned from the response to this disaster that basic knowledge of radiation and its effects is extremely important for not only professionals such as health care providers but also for other professionals including teachers.