Background The Baltic Sea is one of the most polluted, brackish water bodies in the world. The most serious threat is eutrophication caused by high nutrient loading from municipal wastewater, agriculture, scattered settlements, traffic (from nitrogen deposition), energy production and industry. The sea area of Helsinki consists of extensive bays, a fairly sparse archipelago and outlying islets and open sea areas. Seawater quality is affected by impurities in storm waters, diffuse loading, human activities and wastewater, which is fed to the outer archipelago through tunnels. The environmental situation requires local level actors to improve their knowledge and acquire new skills and tools in order to be able to contribute to the protection of the sea. Thus, it is vitally important to use understandable environmental communication and demonstrate the cost-benefit relationship of voluntary water protection measures in order to effectively convey the importance of such actions. The 'CITYWATER' project will be based on the Baltic Sea Challenge Initiative, which will be used as a demonstration basis for the project implementation. The initiative links a network of more than 170 actors, including cities and municipalities, companies, universities and associations. Objectives The overall objective of the project is to implement and facilitate environmentally relevant and cost-effective voluntary water protection measures in cities and municipalities in the Baltic Sea Region in order to improve the state of coastal waters. General working procedures will be improved by increasing environmental communication and knowledge in cities and municipalities in the region in order to ensure continuous work for water quality improvement of local waters. Voluntary water protection work will be promoted and facilitated using the principles of the Baltic Sea Challenge initiative, with the goal of improving the knowledge and communications on Baltic Sea protection of beneficiaries and stakeholders. The project will also assess the environmental impact and cost-benefit ratio of different water protection measures and of different activities that cities and municipalities carry out with regards to the Baltic Sea. This is connected to the need to monitor the impact of project actions and to compile the results from the project with the help of a toolbox. Expected results: Three new investment plans and constructions for storm water handling in Helsinki and Turku in Finland and in Tallinn, Estonia; Tools for more effective environmental communication with a special focus on voluntary water protection measures in Baltic Sea states and within local organisations and on strengthening the network of actors; A ranking of the environmental impact and cost-benefit value of water protection measures implemented in cities and municipalities; Lessons learned from the environmental communication survey and the environmental impact and cost-benefit analyses will be used in the Baltic Sea Challenge after the end of the project; and The Baltic Sea Challenge coordinators will disseminate and, if needed, further develop the results for both old and new actors within the Baltic Sea Challenge network.The results will also be disseminated to national authorities, research institutions, EU bodies, HELCOM and other regions – e.g. the Mediterranean and Black Sea regions can use the results and lessons learned from this project in their water protection work.