Nematodes are slender, worm-like animals, typically less than 2.5 millimetres long. The smallest nematodes are microscopic, while free-living species can reach as much as 5 centimetres and some parasitic species are larger still. The body is often decorated with ridges, rings, warts, bristles or other distinctive structures.

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The head of a nematode is relatively distinctive. Whereas the rest of the body is bilaterally symmetrical, the head is radially symmetrical, with sensory bristles and, in many cases, solid head-shields radiating outwards around the mouth. The mouth has either three or six lips, which often contain a series of teeth on their inner edge. An adhesive gland is often found at the tip of the tail.