Bed bugs are parasitic insects of the cimicid family that feed exclusively on blood. Cimex lectularious, the common bed bug, is the best known as it prefers to feed on human blood. Other Cimex species specialize in other animals, e.g.,  bat bugs, such as Cimex pipistrelli (Europe), Cimex pillosellus (western US), and Cimex adjunctus (entire eastern US).

The name bed bug derives from the preferred habitat of Cimex lectularius: warm houses and especially near or inside beds and bedding or other sleep areas. Bed bugs are mainly active at night, but are not exclusively nocturnal. They usually feed on their hosts without being noticed.