Contamination by bulk plastics and plastic debris is currently the one of the most serious environmental problems in aquatic ecosystems. In particular, small-scale plastic debris such as microplastics and nanoplastics has become leading contributors to the pollution of marine and freshwater ecosystems. Studies are investigating the impacts of micro-and nanoplastics on aquatic organisms and ecosystems worldwide. This review covers 83 studies that investigated the distribution of microplastics and the ecotoxicity of micro- and nanoplastics in marine and freshwater ecosystems. The studies indicated that micro-sized plastics and plastic debris were distributed at various concentrations in aquatic ecosystems around the world. They had various effects on the growth, development, behavior, reproduction, and mortality of aquatic animals. We discuss these studies in detail and suggest directions for future research. •Microplastics and nanoplastics pollute marine and freshwater ecosystems worldwide.•Plastics can affect the metabolism, fertility, and mortality of aquatic organisms.•This review covers 83 studies of micro- and nano-plastics in aquatic ecosystems.•Through trophic transfer, microplastics and nanoplastics can affect humans.•Future work should study interactions between plastic surfaces and the environment.