Tapeworms belong the class of parasitic worms. Most of the species - and the best-known - are those in the subclass Eucestoda; they are ribbon-like worms as adults, known as tapeworms. Their bodies consist of many similar units, known as proglottids, which are essentially packages of eggs which are regularly shed into the environment to infect other organisms. At the front end is the scolex, complete with hooks used to grab onto the internal wall of the intestine they inhabit.

Typically the adults live in the digestive tracts of vertebrates, while the larvae often live in the bodies of other animals, either vertebrates or invertebrates. For example, Diphyllobothrium has at least two intermediate hosts, a crustacean and then one or more freshwater fish; its definitive host is a mammal. Some cestodes are host-specific, while others are parasites of a wide variety of hosts. Some six thousand species have been described; probably all vertebrates can host at least one species.

Humans are subject to infection by several species of tapeworms if they eat undercooked meat such as pork, beef, and fish, or if they live in, or eat food prepared in, conditions of poor hygiene. The unproven concept of using tapeworms as a slimming aid has been touted since around 1900.

Tapeworm head made for the exhibiton "Parasites" on Humboldt Museum für Naturkunde in Berlin.