The central goals of the Fourth Symposium are: (1) To assess recent progress in all aspects of atmospheric and oceanographic data assimilation, in both research and operational environments; and (2) To reach a common understanding of the main challenges and opportunities that lie ahead in atmospheric and oceanographic data assimilation. Contributions will be sought for oral and poster presentations on state-of-the-art research and operational developments in atmospheric and oceanographic data assimilation. While the emphasis will continue to be on atmospheric and oceanic circulation, from global scale to mesoscale, all aspects of the Earth's climate system will be considered, in particular the continental biosphere, stratospheric and tropospheric chemistry, and ocean biogeochemistry. Invited papers will give overviews of problems, progress and prospects in key areas. The Symposium will be held on the premises of Karolinum, the historical main building of Charles University in Prague. Accommodations will be arranged mostly at three-star hotels located in the town center and environs; other hotel categories will be available as well. Local organization is coordinated by the Czech Hydrometeorological Institute, Charles University and the Academy of Sciences of the Czech Republic. Details on the Symposium, including the call for abstracts, registration information, key dates will follow in the Second Announcement and will be published on the web site of the Symposium: http://www.chmi.cz/dasympos/index.html