Freshwater ecosystems were subject to dramatic changes due to human impact in the last decades. As a result, in European inland waters, many indigenous species have declined or disappeared, while other species have considerably extended their species range, thus causing ecological, economic and social damages. In this respect, very good example is the genus Dreissena which includes freshwater and brackish bivalve mollusc species with Ponto-Caspian origin. Two of its representatives the zebra mussel, Dreissena polymorpha and quagga mussel, D. bugensis, are known to be among the most aggressive aquatic invaders. Other representatives, such as D. stankovici, were considered endemic to the ancient tectonic lakes in the Balkan Peninsula. This project aims to contribute to the study of species with genus Dreissena in their native and invaded ranges in Europe. The main objectives are: 1) Compilation of existing data, exchange of experience and knowledge about the highly invasive Dreissena species in Austria, Bulgaria and Romania, and the native species in lakes in Macedonia; 2) Collection of new data on recently infested water bodies and newly established populations; 3) Exchange and integration of different methodological approaches and exchange of research results; 4) Planning and preparing of further collaborative research activities in this area. These objectives will be accomplished by organization of joint field trips in Austria, Bulgaria, Macedonia and Bulgarian-Romanian stretch of the Danube; by organization of working meeting; by preparation of joint publication/s and project draft. With the implementation of this project we expect: to initiate a scientific networking in the area of invasive biology and ecology of Dreissena spp.; to contribute to the development of national, regional and European policy strategies concerning the alien and native invasive species, as well as the preservation of biodiversity and protection of natural ecosystems from invasions; to promote and to obtain support for further actions at national and European level.