[en] IRSN, a public authority with industrial and commercial activities, was set up under Article 5 of French Act No. 2001-398 of May 9, 2001, enacted through Order No. 2002-254 of February 22, 2002. This Order was amended on April 7, 2007. The Institute is placed under the joint authority of the Ministries of Defense, the Environment, Industry, Research, and Health. It is the nation's public service expert in nuclear and radiation risks, and its activities cover all the related scientific and technical issues. Its areas of specialization include the environment and radiological emergency response, human radiation protection in both a medical and professional capacity, and in both normal and post-accident situations, the prevention of major accidents, nuclear reactor safety, as well as safety in plants and laboratories, transport and waste treatment, and nuclear defense expertise. IRSN interacts with all parties concerned by these risks (public authorities, in particular nuclear safety and security authorities, local authorities, companies, research organizations, stakeholders' associations, etc.) to contribute to public policy issues relating to nuclear safety, human and environmental protection against ionizing radiation, and the protection of nuclear materials, facilities, and transport against the risk of malicious acts. This document is the 2011 annual report of IRSN. It presents the Organization, Progress and main activities in 2011, Transparency and communications policy, safety and radiation protection culture of IRSN. The main activities in 2011 are presented in details: 1 - Safety: Safety of existing facilities: Monitoring reactors, Monitoring fuel cycle facilities and experimental reactors, Containment and fire, Natural hazards, Fuels, Accidents, About defense; Conducting assessments of future facilities: Future waste repositories, Reactors of the future; 2 - Security and non-proliferation: Protection against malicious act; Activities of the advisory committees; Assessment and inspection activities; Transport of nuclear materials; Emergency exercises; Security of radioactive sources; International activities; International non-proliferation inspections; 3 - Radiation protection - environment and human health: Environmental and population exposure: Environmental monitoring, Metrology, Polluted sites and soils, Radioecology, Radiation protection in the workplace; Effects of chronic exposure; Protection in health care; 4 - Emergency and post-accident situations: Radiological accidents and incidents; Tools and resources; Nuclear accident at Fukushima-Daiichi; Timeline of IRSN actions. A last part gives some information about IRSN's resources supporting the work of the institute, about its commitment to sustainable development, and about its Human resources. The document ends with a financial and management report