Mollusks are widespread in freshwater ecosystems globally where they can occur at high abundance and play important ecological roles. Alan Covich's broad, pioneering work with freshwater mollusks, including paleolimnology of Yucatan lakes, bivalve and gastropod ecology in springs, streams, and lakes, and the importance of the benthos in provisioning ecosystem services, has informed and inspired our work on the functional roles of freshwater mussels in streams and the ecosystem services they provide. Mussels are powerful suspension feeders that filter particles from the water with their enlarged gills. In doing so, they influence water clarity, recycle nutrients in the water column and transfer energy and nutrients from the water column to the sediment, stimulating production across trophic levels in the aquatic system and subsidizing terrestrial systems. Mussels also are ecological engineers that create and modify habitat for themselves and a multitude of other organisms. Mussels frequently occur in patchily distributed, high abundance, multi-species aggregations (mussel beds). Because of mussels’ filtering and engineering activities, these aggregations can act as hotspots of biological activity in rivers. Freshwater mussels are declining globally, and at higher rates than most other faunal groups. Across the globe, about 45% of freshwater mussels are considered threatened or have gone extinct, and this is likely an underestimate because so many regions and species are underexplored. The complex life history and habits of mussels make them especially susceptible to environmental change, and declines are linked primarily to widespread habitat degradation, hydrologic alteration, pollution, overexploitation, introduction of non-native species and climate change. Declines include not only species’ extinctions but decreased abundance of once common species and the loss of the ecosystem functions and services provided by mussels.