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The leeches are annelids comprising the subclass Hirudinea. There are freshwater, terrestrial and marine leeches. Like their near relatives, the Oligochaeta, they share the presence of a clitellum. Many species of leech are haemophagic parasites, living on occasional meals of blood obtained by attaching themselves to fish, amphibians (frogs etc.), and mammals. The medicinal leech, Hirudo medicinalis, which is native to Europe, has been used for clinical bloodletting for thousands of years. Most leech species are predators and feed on small worms or other invertebrates.

The leeches are clitellate when breeding and are derived from some oligochaete ancestor. They 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 ventral sucker formed from the last six segments of their body, which is used to connect to a host for feeding. They use a combination of mucus and suction (caused by concentric muscles in those six segments) to stay attached. Like earthworms, leeches are hermaphrodites.

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

The leech has long been used in medicine, although today its use is mainly limited to limb reattachment procedures instead of the wide-ranging medical use of the past. Leeches were once so commonly used that doctors were popularly called leeches. In Old High German, lāhhi (etymon of leech) means "physician".

Leech saliva contains a number of compounds which assist in its feeding. An anaesthetic limits the sensations felt by the host (and thus reduces the chance of the host trying to detach the leech). A vasodilator causes the blood vessels near the leech to become dilated, and thus provide the leech with a better supply.

Lastly, the leech saliva contains a complex protein called hirudin, which is a highly effective anticoagulant. The leech needs this to prevent blood clots (which would block its feeding) from forming in the wound created by its mouthparts. These properties are difficult to achieve using other medical techniques, and it is for this reason that leeches have come back into clinical practise in the last 25 years. The small amounts of hirundin present in leeches makes it unsuitable to be harvested for more general medical use, so hirudin (or related chemicals) have been synthesised using recombinant-DNA technology.

Bdellatomy is the practice of cutting the leech open slightly while it is sucking blood to let the blood in it out, so, thinking that it is not full yet, the leech continues to bite instead of detaching itself. This practice was first recorded in 1868 by Daily News.

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.

Not much can be done to prevent leech bites in the outdoors. There is little evidence in favour of any leech-repellant (unlike the strong evidence in favour of DEET against biting insects). The most effective step is not to expose bare skin, preferably by wearing leech socks.

Leech socks, calico oversocks worn outside the socks and trousers provide a degree of protection, since they prevent leech entry at ankle level. The pale material from which they are usually made makes it easy to spot leeches as they climb looking for the next area of bare skin on the arms or neck.

It is common practice for people leaving leech-infested waters or wet vegetation to conduct an inspection of themselves to ensure that no leeches are attached.

If a leech bites, it should be removed and the wound cleaned. There is an urban legend that if the biting parts of the leech are left in the wound, healing is inhibited. There is no evidence in favour of such an assertion. Leeches can most easily be removed by sprinkling salt on them (which irritates their skin, causing them to flee) or by (carefully) burning them with a lit cigarette, heated paperclip, smoldering piece of grass or small stick or match (which again causes the leech to disengage its mouthparts and flee).

Edible lime (Chuna in India) diluted in water can be sprayed either as a deterrent or onto the animal when it is attached. The solution causes the leech to disintegrate. There are doubts as to whether this is a safe practice, since the metal oxide/hydroxides concerned may damage the skin.

There appears to be little evidence of transmission of diseases through leech bites. Leeches are very distantly related to those organisms which carry rabies or other diseases affecting people, so disease-carrying organisms do not breed within a leech; in addition, they need to feed so infrequently that pathogens would die before being transmitted. If the wound is cleaned, there is little risk of infection (as with any other small wound).

The most dangerous practice seems to be scratching leech wounds with the fingernails, and hence acquiring other infections. People living in leech infested areas take getting some bites as a matter of course. Although the bites are painless and generally harmless, many people are squeamish about these creatures, and the prospect of removing blood-soaked socks.