The head louse (Pediculus humanus capitis) is an obligate ectoparasite of humans. Head lice are wingless insects spending their entire life on the human scalp and feeding exclusively on human blood. Like all lice, head lice differ from other hematophagic ectoparasites such as the flea in that lice spend their entire life cycle on a host. Head lice cannot fly, and their short stumpy legs render them incapable of jumping, or even walking efficiently on flat surfaces.

The non-disease-carrying head louse differs from the related body louse in preferring to attach eggs to scalp hair rather than to clothing. A much more distantly related species of hair-clinging louse, the pubic louse (crab louse), also infests humans. Lice infestation of any part of the body is known as pediculosis.

Head lice are not the vectors of any known diseases, except for rare secondary infections that result from scratching at bites. Head lice are harmless, and have been regarded as essentially a cosmetic rather than a medical problem.