The best known and currently, most popular interpritation of a intelegent alien race.
Although mostly humanoid in shape, their bodies seem to have evolved into a shape more fitting for a spieces of higher intelegence. Described as having larger heads, and small bodies that lack muscle definition. Little else seems to be constant about the specie's discriptions. Eyewhitnesses describing them having every thing from small mouths or no mouths, excessivly long and lanky limbs or short and stubby limbs, textured skin or smooth skin, clothing or "naked"; the list of conflicting traits goes on.
One theory to this is that there is more than one breed of Greys (Or what ever color their skin is in the corrisponding story). Another theory is that they can shape-shift, or appear to the contactee as what ever said contactee would expect them to look like. Possibly, the specie's body is an illusion and actually have no physical form, but use a physical form so as not to confuse the contactee. This may also explain why many Cryptid's descriptions tend to conflict from one eyewhitness to another.
Another common trait between reports is that the species has telepathic abilities and communicate through ESP. This may explain why the may descriptions of the creatures have small mouths, lack lips, or don't have mouths at all (As gaining telepathic abilities would mean that a mouth wouldn't need to be used for speech, and thus useless traits would have evolved out).
I personally agree that the species may have shape-shifting abilities. Weather or not they employ it to strike fear into their contactees or use it to give themselves a more acceptible form for their contactee to understand is still a mystery. It is realistic though. If you look at the reports from the 50's onword, the originally looked more or less like humans; but their shape changed as more and more rumors and reports surfaced about stranger looking creatures.