The Magnitude 9 Great East Japan Earthquake on March 11, 2011, followed by a massive tsunami struck TEPCO's Fukushima Daiichi Nuclear Power Station and triggered an unprecedented core melt/severe accident in Units 1 - 3. The radioactivity release led to the evacuation of local residents, many of whom still have not been able to return to their homes. As a group of nuclear experts, the Atomic Energy Society of Japan established the Investigation Committee on the Nuclear Accident at the Fukushima Daiichi Nuclear Power Station, to investigate and analyze the accident from scientific and technical perspectives for clarifying the underlying and fundamental causes, and to make recommendations. The results of the investigation by the AESJ Investigation Committee has been compiled herewith as the Final Report. Direct contributing factors of the catastrophic nuclear incident at Fukushima Daiichi NPP initiated by an unprecedented massive earthquake/ tsunami - inadequacies in tsunami measures, severe accident management, emergency response, accident recovery and mitigations - and the underlying factors - organizational issues, etc., have been clarified and recommendations in the following areas have been made. - Nuclear safety fundamentals - Direct factors of the accident - Organizational aspects - Common items (R&D, International cooperation, human resources management) - Post-accident management/recovery from the accident.