There is growing evidence that free-living freshwater nematodes, previously largely neglected, play important roles in ecosystem function. In freshwater sediments, nematodes are usually the most abundant organismal group of metazoans, and they are found from the polar regions to the tropics. In lakes and rivers, nematodes dominate the meiobenthos, which comprises benthic animals retained on a net with a mesh size of about 40 μm and passing through a mesh size of 0.5–1 mm. Only four groups of animals (nematodes, spiders, insects, and vertebrates) are found in nearly all habitats (including sea muds, seas, freshwater muds, freshwaters, terrestrial habitats), and they live as ecto- and endoparasites. If there are so many nematodes and so many different kinds, why are they so seldom seen by people? The common forms that live in water are usually too small to be observed by ordinary methods of visual examination. Moreover, most nematodes are ‘buried’ in their habitat, whether ocean, freshwater, soil, or tissues of animals and plants. The classification systems of most nematodes are based on relatively few morphological characteristics, derived primarily from light microscopic observations. The advent of molecular techniques has brought substantial changes to the strengths and limitations of nematode systematics. Despite the almost ubiquitous distribution of nematodes and their diverse ecological roles, the global and local distribution, species composition, and functional roles of these organisms in most freshwater habitats are still unknown. The role of nematodes in the food web seems to be twofold: (1) the uptake of bacteria and other small particulate organic carbon, and (2) the transfer of their own biomass to other benthic organisms and young fishes.