The spread of invasive species represents one of the main causes of biodiversity loss in the world. This work describes the harmful effects of invasive molluscs, bivalves and snails, for native species, habitat and ecosystem as a whole. In terms of environmental and socio-economic effects, Dreissena polymorpha is one of the most destructive species in aquatic ecosystems. It causes numerous adverse effects such as deflection of indigenous bivalves, water quality changes, changes in the structure of benthos and fouling vessels and pipelines. Three other species of invasive bivalves have been described: Mytilopsis leucophaeata whose fouling causes great harm to the industry, Limnoperna fortunei who has detrimental acts to the indigenous bivalves, reducing stocks of phytoplankton and polluting the water systems of various plants. Corbicula fluminea has similar environmental effects as D. polymorpha - a negative impact on indigenous Unionidae, by reducing phytoplankton because of the high rate of filtration and by fouling cooling systems in power plants. Four species of snails are also referred as species of harmful effects: Pomacea canaliculata and P. insularum, by threatening native species and destroying rice crops, which contribute to large economic loss of the area, and in addition can be vectors of disease and endanger human health. Bithynia tentaculata infests water supply systems, reduces the number of native species and transmittes trematodes that infest and kill waterfowl. Potamopyrgus antipodarum supersedes native bivalves and changes primary production with dense populations, which has significant effects on the ecosystem. One of the biggest problems in combating the spread of invasive species is late detection of harmful effects after the introduction, and therefore it is important to early detect the potentially invasive species and to apply the methods of reducing their negative impact on the indigenous fauna.