This paper provides an overview of current knowledge regarding the aquatic effects of nanomaterials. Aquatic receptors can potentially be exposed to nanoparticles through ingestion, movement across gills, passive transport, and cellular absorption. Our review indicates that the toxicological research on nanomaterials is still relatively narrow. The present research has been targeted primarily at understanding potential effects to humans, and relatively few ecotoxicity studies have been conducted; however, new research is evolving rapidly. The studies that are currently available focus on metal oxide particles, carbon nanotubes, and fullerenes. Aquatic tests have examined the uptake of these nanoparticles by fish and filter feeders, and have provided evidence of toxicity or behavioral changes. Some of these studies conclude that nanoparticles can be taken up by or produce effects in biota, and that dose-response relationships and patterns of relative toxicity among types of particles are emerging. Caution should be used in designing and interpreting studies on nanoparticles, because factors such as the particle medium preparation method, the presence of other chemicals, and particle behavior such as agglomeration can influence exposure and aquatic toxicity. Ultimately, the objectives of this review are to expand our knowledge of the effects of nanomaterials on ecological processes and aquatic receptor populations, as well as help guide future research.