SYNOPSIS. The expansion of zebra mussel distribution into inland waterways of North America portends significant abiotic and biotic changes mediated either directly or indirectly by Dreissena. Dreissena fouls a wide array of submerged substrates including rock surfaces, macrophytes, native molluscs, canal and dock walls, and watercraft and motor outdrives. Fouling of water intake pipes and associated installations can severely impair water delivery to hydroelectric, municipal and industrial users, necessitating proactive or reactive control measures. Mussels increase water clarity by removing suspended clay, silt, bacteria, phytoplankton, and small zooplankton. Clear water phases associated with Dreissena grazing may exceed in magnitude and duration those generated by zooplankton grazing. Enhanced water clarity increases light transmittance and growth of benthic plants. Some benthic invertebrates {e.g., unionid molluscs) are adversely affected by Dreissena, whereas others, including amphipod crustaceans, exploit structure associated with or wastes generated by zebra mussels. Dreissena is exploited by a host of predators, most notably waterfowl, fish and crayfish. Waterfowl predators that consume contaminated Dreissena have elevated concentrations of organic pesticides and polychlorinated biphenyl compounds. Invasion of shallow lakes and ponds by Dreissena may divert production and biomass from pelagic to benthic foodwebs, shifting ecosystems to an alternative state.