On March 11, 2011, the Tohoku earthquake generated a huge tsunami that ravaged the Pacific coast of Japan, leading to the Fukushima Daiichi nuclear power plant (NPP) accident. Three cores out of the six Fukushima Daiichi nuclear reactors melted, resulting in the release of huge amounts of radionuclides into the atmosphere. Two years after the accident, many uncertainties remain, which prevent us from reaching a full understanding of the accident. This article presents the approach carried out during the accident and afterwards, to improve our understanding of the events and environmental consequences. We trace the time evolution of the radioactivity released to the atmosphere, its subsequent dispersion simulated by models, and we compare the results to actual measurements. Four main release periods are highlighted. The first event had limited consequences to the north of the NPP along the coast; the second had no impact on Japanese territory because the plumes traveled toward the Pacific Ocean; the third was responsible for a significant and long-term impact, especially northwest of the NPP; and the last had consequences of lesser impact on the Tokyo area. The source term, i.e. the release rate of each radionuclide, is still highly uncertain. To improve it, a new method based on inverse modeling techniques and involving the use of gamma dose rate measurements has been developed. The method proved to be efficient and reliable when applied to the Fukushima accident. The inverted source term allowed identifying the main contamination events and helped to improve the model-to-data comparisons. An important outcome on this study is that the method proved to be perfectly suited to crisis management. It should contribute to improve the emergency response of the crisis center in case of a nuclear accident.