Trematodes are also known as flukes. They form a class within the phylum Platyhelminthes.

All trematodes are parasitic. Previous classifications divided them into two orders (or two subclasses), the Monogenea and the Digenea, on the basis of whether their life cycle involved one or two hosts. However the taxonomy of the Platyhelminthes is being subjected to extensive revision. Modern sources place the Monogenea as a separate class within the phylum.

Two types of fluke infections occur in humans. One involves tissue flukes, which attach to the bile ducts, lungs, or other biological tissues; the other involves blood flukes which are found in the blood in some stages of their life cycle. Tissue flukes that parasitize humans include the lung fluke, Paragoniumus wetermani, and the liver flukes, Clonorchis sinensis and Fasciola hepatica. Blood flukes include various species of the genus Schistosoma.

Parasitic flukes have a complex life cycle, often involving several hosts. The fusion of male and female gametes produces fertilized eggs that become encased in tough shells during their passage through the female fluke's uterus. The eggs pass from the host with the feces. When the eggs reach water, they hatch into free-swimming forms called miracidia. The miracidia penetrate a snail or other molluscan host, become sporocysts, and migrate to the host's digestive gland. The cells inside the sporocysts typically divide by mitosis to form rediae. Rediae, in turn, give rise to free-swimming cercariae, which escape from the mollusk into water. Using enzymes to burrow through exposed skin, cercariae penetrate another host (often an arthropod) and then encyst as metacercariae. When this host is eaten by the definitive host, the metacercariae excyst and develop into mature flukes in the host's intestine.

See Microphallus piriformes for an example of a fluke that modifies the behaviour of one of its hosts.