This report outlines the response of the OECD Nuclear Energy Agency (NEA) and its member countries to the March 2011 accident at TEPCO's Fukushima Daiichi nuclear power plant. All NEA members took early action to ensure and confirm the continued safety of their nuclear power plants and the protection of the public. Consistent with its objective of maintaining and further developing the scientific, technological and legal bases for safe nuclear energy, the NEA has assisted its member countries in their individual and collective responses to the accident. It has also provided direct assistance to the relevant authorities in Japan. These actions are summarised in the report along with lessons learnt thus far. Key messages are offered as a means to help strengthen the basis for nuclear safety and its implementation in all countries using nuclear power.