A beard is the hair that grows on the jaw, chin, upper lip, lower lip, cheeks, and neck of humans and some non-human animals. In humans, beards are most common among pubescent and adult males, though some women also develop them.

Attitudes toward beards have varied across history, shaped by cultural traditions and fashion trends. Several religions require or encourage the wearing of beards, while other societies have associated them with masculinity, virility, virtue, beauty, wisdom, strength, fertility, sexual prowess, and high social status. In contrast, in cultures where beards are uncommon or unfashionable, they may be linked with poor hygiene or eccentricity. Beards can also provide environmental benefits, including protection from cold weather and sun exposure.

The beard develops in human males during puberty. Beard growth is linked to stimulation of hair follicles in the area by dihydrotestosterone, which continues to affect beard growth after puberty. Dihydrotestosterone also promotes balding. Dihydrotestosterone is produced from testosterone, the levels of which vary with season. Beard growth rate is also genetic.

Biologists characterize beards as a secondary sexual characteristic because they are (mostly) unique to one sex, yet do not play a direct role in reproduction. Charles Darwin first suggested a possible evolutionary explanation of beards in his work The Descent of Man, which hypothesized that the process of sexual selection may have led to beards. Modern biologists have reaffirmed the role of sexual selection in the evolution of beards, concluding that there is evidence that a majority of women find men with beards more attractive than men without beards.

Evolutionary psychology explanations for the existence of beards include signalling sexual maturity and signalling dominance by the increasing perceived size of jaws; clean-shaved faces are rated less dominant than bearded. Some scholars assert that it is not yet established whether the sexual selection leading to beards is rooted in attractiveness (inter-sexual selection) or dominance (intra-sexual selection). A beard can be explained as an indicator of a male's overall condition. The rate of facial hairiness appears to influence male attractiveness. The presence of a beard makes the male vulnerable in hand-to-hand fights (it provides an easy way to grab and hold the opponent's head), which is costly, so biologists have speculated that there must be other evolutionary benefits that outweigh that drawback. Excess testosterone evidenced by the beard may indicate mild immunosuppression, which may support spermatogenesis.

Phoenicia, the ancient Semitic civilization centered on the coastline of the Eastern Mediterranean (modern-day Syria, Lebanon and Israel), gave great attention to the hair and beard. It was arranged in three, four, or five rows of small tight curls, and extended from ear to ear around the cheeks and chin. Sometimes, however, in lieu of the many rows, we find one row only, the beard falling in tresses curled at the extremity. There is no indication of the Phoenicians having cultivated mustachios.

Israelite society placed a special importance on the beard. Many male religious figures mentioned in the Tanakh are recorded to have had facial hair. According to biblical scholars, the shaving of hair, particularly of the corners of the beard, was a mourning custom. The religious cultivation of beards by Israelites may have been done as a deliberate attempt to distinguish their behaviour in comparison to their neighbours, reducing the impact of foreign customs (and religion) as a result. The Hittites and Elamites were clean-shaven, and the Sumerians were also frequently without a beard; conversely, the Egyptians and Libyans shaved the beard into very stylised elongated goatees.

Mesopotamian civilizations (Sumerian, Assyrians, Babylonians, Chaldeans and Medians) devoted great care to oiling and dressing their beards, using tongs and curling irons to create elaborate ringlets and tiered patterns.