They are found from the bottom of the deepest ocean to near the tops of the highest mountains, from the tropics to polar regions, and from every conceivable habitat. It is hoped this key will fill some of this void. Nematodes range in size from nearly 10 metres long to only a fraction of a millimetre. In diameter, they range from a few millimetres to about 5 microns. Although they are small, nematodes are incredibly abundant.
Nematodes are even more abundant in terrestrial, estuarine and marine substrates.
Illustrated at right is the effect of plant parasitic nematodes on a canola crop. Because of their ubiquity and diversity, nematodes are also used in measuring the impacts of various perturbations on ecosystems, such as pollution, organic enrichment, and physical disturbance. Nematodes are intimately involved in many parts of the soil ecosystem, so they can be used as indicators of sustainability for soils. It is hoped that this identification system will assist both those studying nematodes as part of other problems, as well as those studying them in their own right. Juveniles, adult females, adult males, and populations of each genus are coded separately.