Nematodes are probably the most abundant metazoans in the biosphere and of very great importance to man. As parasites of man they are responsible for disease in hundreds of millions of people and an estimated eight billion nematodes are enjoying food, warmth and shelter in human intestines today; Ascaris lumbricoides is, after the viruses responsible for diarrhoeal disease, the second most infectious organism in the world, about a quarter of the world's population being infected (Ash, Crompton & Keymer, 1984). Plant-parasitic nematodes cause considerable damage to crops and many species are vectors of soil-borne viruses. Today, plant nematology is one of the most vital fields in agricultural research and is studied in many university departments over the world, yet fifty years ago only a few pioneers were engaged in the study of plant-parasitic nematodes and were convinced of the economic importance of these worms. Whereas the importance of parasitic nematodes has now been recognized for many decades, this is not the case for the free-living species, especially those of aquatic environments. They remain relatively unstudied, despite the fact that they are extremely abundant, often numbering millions per m² in soils and sediments, and that they occur in a range of habitats which is unsurpassed by any other metazoan group, being absent only from the oceanic plankton. Free-living nematodes are small and inconspicuous and have very rarely attracted amateur naturalists. They tend to live in environments such as intertidal muds that are not particularly appealing to many people. Many species are difficult to maintain in the laboratory and the taxonomic literature has been very scattered until quite recently, making determination of nematodes a frightening affair and putting ecologists in the embarrassing position of studying animals they could not even name. Yet, other fields have recently been attracted to nematodes, and species such as the small soildwelling Caenorhabditis have become very popular model organisms in molecular biology, genetics (Brenner, 1974), and even gerontology (Vanfleteren, 1978), where methods have been developed that greatly surpass the degree of sophistication reached in most ecological work. The lack of interest for a group that is as dominant in sediments as copepods are in the plankton seems no longer justified on methodological grounds. It is also unjustified on ecological grounds. Nematodes are important in the ecology of the seas and they are interesting. Of course, they are small or even very small (down to 100 µm adult size) and structurally simple, basically consisting of two concentric tubes. In general most species are slender, with only a few tenths of pm diameter, but a considerable variation in habitus exists. As Platt & Warwick (1980) observed, not all nematodes look alike, many species of fine sediments are short while in coarse sands species are either very small or very elongate and thin. They are inconspicuous; many older papers do not even mention the occurrence of nematodes in sediments as nearly all specimens disappeared through the sieves that were used. Even today, when sieves of around 50 µm mesh size are universally used, estimates of nematode abundance are biased since many of the small species and juveniles of the bigger ones will be lost while processing the sample. Despite their similar basic morphology nematodes occupy very different roles and trophic positions in sediments. Many species feed on bacteria, on algae or on both, they eat detritus and possibly dissolved organic matter, and a considerable number are predators, feeding on other nematodes, oligochaetes, polychaetes, etc. This diversity in feeding is reflected in species diversity; the number of nematode species in most habitats is much higher than that of any other metazoan group. As an example, a recent review counted a total of 735 nematode species in the North Sea (Heip, Herman & Vincx, 1983) and it is not uncommon to obtain 50 species or even more in a single 10 cm² core, often many congenerous, a situation that should be appealing to the theoretical ecologist trying to explain community structure in terms of predation or competition. That nematodes are important in the energy flow through sediments has often been inferred from their numerical abundance. Yet, reliable data are scarce. In this paper we shall try to evaluate the existing literature, both the rather extensive literature describing the main structural features of nematode communities in the sea, and the far fewer papers on functional relationships and their possible use in pollution studies.