Leeches are annelids comprising the subclass Hirudinea. There are freshwater, terrestrial and marine leeches. Like earthworms, leeches are hermaphrodites. The medicinal leech, Hirudo medicinalis, which is native to Europe, and its congeners have been used for clinical bloodletting for thousands of years.

All leech species are carnivorous and have evolved from detritus-eating earthworms/oligochaete ancestors. Some are predatory, feeding on a variety of invertebrates such as worms, snails, insect larvae, crustaceans, while a very few are blood-sucking leeches, feeding on the blood of vertebrates such as amphibians, reptiles, waterfowl, fish, and mammals. Given the opportunity, they will also feed on human blood. The most important predators on leeches are fish, aquatic insects, crayfish and other leeches.

Haemophagic leeches attach to their hosts and remain there until they become full, at which point they fall off to digest. Leeches' bodies are composed of 34 segments. They all have a anterior (oral) sucker formed from the last six segments of their body, which is used to connect to a host for feeding, and are known to release an anaesthetic to remain unnoticed by the host. They use a combination of mucus and suction (caused by concentric muscles in those six segments) to stay attached and secrete an anti-clotting enzyme into the host's blood stream.

Some species of leech will nurture their young, providing food, transport, and protection, which is unusual behavior in an invertebrate.

The anatomy of medicinal leeches may look simple, but more details are found beyond the macro level. Externally, medicinal leeches tend to have a brown and red striped design on an olive colored background. These organisms have two suckers, one at each end, called the anterior and posterior sucker. The posterior is mainly used for leverage while the anterior sucker, consisting of the jaw and teeth, is where the feeding takes place. Medicinal leeches have three jaws that look like little saws, and on them are about 100 sharp teeth used to incise the host.

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