Digenea (Gr. Dis – double, Genos – race) is a class of trematodes in the Platyhelminthes phylum, consisting of parasitic flatworms (known as flukes) with a syncytial tegument and, usually, two suckers, one ventral and one oral. Adults commonly live within the digestive tract, but occur throughout the organ systems of all classes of vertebrates. Once thought to be related to the Monogenea, it is now recognised that they are closest to the Aspidogastrea and that the Monogenea are more closely allied with the Cestoda. Around 6,000 species have been described to date.

Characteristic features of the Digenea include a syncytial tegument; that is, a tegument where the junctions between cells are broken down and a single continuous cytoplasm surrounds the entire animal. A similar tegument is found in other members of the Neodermata; a group of platyhelminths comprising the Digenea, Aspidogastrea, Monogenea and Cestoda. Digeneans possess a vermiform, unsegmented body-plan and have a solid parenchyma with no body cavity (coelom) as in all platyhelminths.

There are typically two suckers, an anterior oral sucker surrounding the mouth, and a ventral sucker sometimes termed the acetabulum, on the ventral surface. The oral sucker surrounds the mouth, while the ventral sucker is a blind muscular organ with no connection to any internal structure.

A monostome is a worm with one sucker (oral). Flukes with an oral sucker and an acetabulum at the posterior end of the body are called Amphistomes. Distomes are flukes with an oral sucker and a ventral sucker, but the ventral sucker is somewhere other than posterior. These terms are common in older literature, when they were thought to reflect systematic relationships within the groups. They have fallen out of use in modern digenean taxonomy.

The vast majority of digeneans are hermaphrodites. This is likely to be an adaptation to low abundance within hosts, allowing the life cycle to continue when only one individual successfully infects the final host. Fertilisation is internal, with sperm being transferred via the cirrus to the Laurer's Canal or genital aperture. A key group of digeneans which are dioecious are the schistosomes. Asexual reproduction in the first larval stage is ubiquitous.

While the sexual formation of the digenean eggs and asexual reproduction in the first larval stage (miracidium) is widely reported, the developmental biology of the asexual stages remains a problem. Electron microscopic studies have shown that the light microscopically visible germ balls consist of mitotically dividing cells which give rise to embryos and to a line of new germ cells that become included in these embryonic stages. Since the absence of meiotic processes is not proven, the exact definition remains doubtful.

Protandry is the general rule among the Digenea. Usually two testes are present, but some flukes can have more than 100. Also present are vasa efferentia, a vas deferens, seminal vesicle, ejaculatory duct and a cirrus (analogous to a penis) usually (but not always) enclosed in a cirrus sac. The cirrus may or may not be covered in proteinaceous spines. The exact conformation of these organs within the male terminal genitalia is taxonomically important at the familial and generic levels.

Usually there is a single ovary with an oviduct, a seminal receptacle, a pair of vitelline glands (involved in yolk and egg-shell production) with ducts, the ootype (a chamber where eggs are formed), a complex collection of glands cells called Mehlis’ gland, which is believed to lubricate the uterus for egg passage.