In Ufology, Greys, also known as Roswell aliens, Zetas, and Reticulians, are alleged extraterrestrial life forms that appear in modern UFO conspiracy theories and other UFO-related paranormal phenomena. They make up approximately 75 percent of all aliens reported in the US, 20 percent of all aliens reported in Mainland Europe, and 12 percent of all aliens reported in Britain. In American popular culture they have replaced little green men as the most typical extraterrestrial life form.

Typically, Greys are described as being approximately 120 cm tall, with grey (sometimes blue-grey/green-grey) skin. Their body is typically described as being elongated, and lacking in muscular definition. Their legs are shorter and jointed differently than one would expect in a human, giving them an apparently awkward gait. Their arms often reach down to their knees, and some accounts give them three digits, or three digits and a thumb on each hand. They have a bulbous, hairless head supported by a thin neck, which is dominated by large, (usually black) lidless eyes. They typically have small flat noses, small mouths and small ears lacking a pinna. In some cases, Greys are said to have slit-like nostrils on a flat face.

Some accounts have Greys wearing tight neutral colored uniform-like jumpsuits. Other reports have them appearing to be naked. In most cases, clothed Greys have no determinable gender and naked Greys have no visible external genitals.