An event most often associated with Grey aliens is the Roswell Crash in New Mexico in summer 1947. Details of this event are difficult to tease out from the masses of official and unofficial material on the subject, which tend to extremes of conspiracy theory and of official cover-up. It is very difficult to tell which documents and accounts of this event are real, which are fake and which are just out of context, and range from a theory that the US government was using saucer-shaped weather balloons to test for a surveillance program, only to be foisted by its own radar patard, to the USSR deliberately sending balloons with child-sized dummies in them in an effort to cause hysteria in the US, to actual aliens.
Greys have commonly been described as humanoid beings with hairless grey skin (hence the name), standing between 2-4 feet tall, sporting bulbous heads and large black eyes. They have also been frequently described as being thin and frail and never wearing clothing. They have no visible ears or sexual organs, tiny noses and short mouthes.
If Greys do exist and have truly been visiting Earth, no one knows precisely why. The theories behind their visits are many and range between simple curiosity to more outrageous, even incomprehensible reasons. In many tales of alien encounters, Greys have supposedly abducted human beings and different kinds of animals for experimentation. The suspected reasons behind these experiments include bizarre attempts at communication, genetic tampering (perhaps to determine if Greys can breed with humans or to use Earth life as material for creating biological weaponry), or to determine if Earth life is suitable for them to ingest. Other theories regarding the Greys' observation of Earth include studying humans to determine how threatening we may be and whether or not our planet is worth conquering. The debate of Grey existence and their plans among modern culture continues and will likely go on until the aliens - if they do exist - finally decide to open communications with humanity.