On 26 April 1986, the worst nuclear accident in the history of humankind occurred at the Chernobyl nuclear power plant accident in Ukraine. The disaster that was rated highest at the International Nuclear Event Scale (INES) and affected the environment and the lives of millions of people in Belarus, Ukraine and the Russian Federation. Evacuation and relocation proved a deeply traumatic experience to many people with profound psychosocial impact due to the loss of homes and jobs, rupture of social fabric of communities and a social stigma associated with Chernobyl. WHO, along with the scientific research agencies and the governments of the three affected states, has been leading the efforts to assess and mitigate the health consequences of the accident through limiting the exposure of the population, supporting medical follow-up to those affected and harnessing the scientific cooperation to study the health impact of the incident.