"Greys" are allegedly extraterrestrial beings whose existence is popularly promoted in ufological, paranormal, and New Age circles despite being disregarded by the mainstream scientific community. Named for their alleged skin tone, Greys are most widely associated with the alien abduction phenomenon, wherein claimants allege that Greys are intelligent extraterrestrials who visit Earth and secretly perform medical experiments on humans they have temporarily kidnapped. Scientists believe that the alien abduction phenomenon is a subjectively real experience with roots in psychology and culture, but that it does not provide credible evidence for visiting extraterrestrials.

Paranormal and pseudoscientific claims involving Greys vary in every respect including their nature (ETs, extradimensionals, demons, or machines), origins, moral dispositions, intentions, and physical appearances (even varying in their eponymous skin color). A composite description derived from overlap in claims would have Greys as small bodied, sexless beings with smooth grey skin, enlarged head and large eyes. The origin of the idea of the Grey is commonly associated with the Betty and Barney Hill abduction claim, although skeptics see precursors in science fiction and earlier paranormal claims.