A lamprey (sometimes also called lamprey eel) is a jawless fish with a toothed, funnel-like sucking mouth, with which most species bore into the flesh of other fish to suck their blood. In zoology, lampreys are often not considered to be true fish because of their vastly different morphology and physiology.

Lampreys begin life as burrowing freshwater larvae (ammocoetes). At this stage, they are toothless, have rudimentary eyes, and feed on microorganisms. This larval stage can last five to seven years and hence was originally thought to be an independent organism. After these five to seven years, they transform into adults in a metamorphosis which is at least as radical as that seen in amphibians, and which involves a radical rearrangement of internal organs, development of eyes and transformation from a mud-dwelling filter feeder into an efficient swimming predator, which typically moves into the sea to begin a predatory/parasitic life, attaching to a fish by their mouths, secreting an anticoagulant to the host, and feeding on the blood and tissues of the host. In most species this phase lasts about 18 months. Whether lampreys are predators or parasites is a blurred question.