Cestoda is one of the four classes of the phylum Platyhelminthes (flatworms). The members of this class are referred to as cestodes or by their common name, tapeworms. They are all parasites that infest vertebrate hosts.

The tapeworms (cestodes) are nearly all segmented, which means that they are built up of individual thin, flat segments, called proglottids, that are connected together to form a long, tape-like body. One end of the body is the scolex, a specialized structure for anchoring the worm to the intestinal wall of the host animal. The scolex is equipped with four suckers and a cluster of tiny hooks. These worms have no mouths or digestive tracts; nutrients are absorbed through the cuticle, or "skin". The worm grows by budding of the segments and can reach lengths of from three to fifteen meters, depending on the species.

Each segment (proglottid) is a reproductive unit. As the segment grows and matures, it develops male and female reproductive organs. A segment can fertilize itself or cross-fertilization can occur between adjacent segments of the same worm or between the segments of different worms. The gravid segments may fall off and leave the host via the anus or they may discharge eggs which pass out with the host's feces. Discharged egg embryonize in water and are ingested by a chain of progressively larger animals up to fish that are large enough to be eaten by a potential host. Thus the cestode life cycle continues.

The cestodes parasitic to man are Taenia saginata (beef tapeworm), Taenia solium (pork tapeworm), Hymenolepsis nana (dwarf tapeworm) and Diphyllobothrium latum (fish tapeworm). The fish tapeworms are the longest, averaging 10 m. The adult tapeworms live in the intestines, where, aside from the major grossness of having a belly full of worms, they don't do all that much damage to the host. Major infestations can result in weight loss by the host.

The larval form of the worm, however, can migrate and invade nearly any tissue in the host's body, including the brain. This condition is called cysticercosis.