This group contains many of the most pathogenic tapeworms of humans, domestic and wild animals.
Adults live in the intestines of all major vertebrate groups except fishes. Gravid proglottids are shed and the eggs must be eaten by a suitable intermediate host, which is an invertebrate or vertebrate. Most species have just a single intermediate host. A variety of larval forms occur in the intermediate host: cysticercoid, cysticercus, coenurus, strobilocercus, and hydatid are the common ones. Humans are definitive hosts to a few species, but can be intermediate hosts to many others.
Many species are small, just a few mm long, but others can reach lengths of 30 m. The scolex has four large suckers and may also have an apical rostellum, often adorned with hooks or spines. Genital pores are lateral, and the vitellaria are located posterior to the ovary, usually in a compact mass. There may be multiple sets of reproductive organs in each proglottid.