Cimex is a genus of insects in the family Cimicidae. Cimex species are ectoparasites that typically feed on the blood of birds and mammals. Two species, Cimex lectularius and Cimex hemipterus, are known as bed bugs and frequently feed on humans, although other species may parasitize humans opportunistically. Species that primarily parasitize bats are known as bat bugs.

The Cimicidae are a family of small parasitic bugs that feed exclusively on the blood of warm-blooded animals. They are called cimicids or, loosely, bed bugs, though the latter term properly refers to the most well-known member of the family, Cimex lectularius, the common bed bug and its tropical relation Cimex hemipterus. The family contains over 100 species. Cimicids appeared in the fossil record in the Cretaceous period. When bats evolved in the Eocene, Cimicids switched hosts and now feed mainly on bats or birds. Members of the group have colonised humans on three occasions.