British ufologist 
Nick Redfern discussed his research into the Men in  Black (MIB) phenomenon, in which he looked at numerous cases from the  start of UFO sightings to the present day. The first report of Men in  Black was associated with Albert Bender, who was studying UFOs in the  early 1950s and formed a popular group called the International Flying  Saucer Bureau. But then, he suddenly shut the organization down after he  was visited by three men wearing black suits who told him that he was  getting too close to the truth, and if he knew what was good for him,  he'd leave the subject alone. Later, researcher Gray Barker popularized  Bender's tale, and implied that the Men in Black were government agents,  Redfern detailed.
In many of the MIB accounts, witnesses  describe the visitors as having somewhat odd appearances and behavior,  including bulging eyes, being very short, sometimes lacking in emotion,  and suddenly disappearing. Their goal always seems to be intimidation,  which is typically accomplished with veiled threats, said Redfern, who  added that he found it curious that people invariably allowed these  strangers into their homes, which indicates the MIBs might employ a kind  of hypnosis.
Redfern has concluded there may be more than one  group behind the MIB phenomenon. For instance, there is documented  evidence that government agents have visited people who've reported UFO  sightings, yet some of the paranormal aspects to MIB visitations suggest  there may also be alien/hybrids making their own visits to determine  how much people know about them. Joshua P. Warren proposed an  interesting theory to Redfern-- that MIBs may actually be time  travelers. The reason they wear the black suits is because this look  will fit in in many time periods, and this might also explain why the  cars they drive are sometimes out of date, Warren said. In reference to  photo documentation of MIBs, Redfern mentioned this photo taken by  Timothy Green Beckley.
Biography:
Nick Redfern has been  interested in UFOs since 1978. His main area of research centers around  determining what has been learned about the UFO subject at an official  level in Britain. He has spent hundreds of hours at the Public Record  Office in London, and has uncovered thousands of pages of  previously-classified Royal Air Force, Air Ministry and Ministry of  Defense files on UFOs dating from the Second World War. Nick is the  author of several best-selling books on UFOs including: The FBI Files:  The FBI's UFO Top Secrets Exposed; and Cosmic Crashes: The Incredible  Story Of The UFOs That Fell To Earth. Nick also lectures on the UFO  subject both in the U.K. and abroad.
Wikipedia
Men in Black  (MIB), in American popular culture and in UFO conspiracy theories, are  men dressed in black suits who claim to be government agents who harass  or threaten UFO witnesses to keep them quiet about what they have seen.  It is sometimes implied that they may be aliens themselves. The term is  also frequently used to describe mysterious men working for unknown  organizations, as well as to various branches of government allegedly  designed to protect secrets or perform other strange activities. The  term is generic, used for any unusual, threatening or strangely behaved  individual whose appearance on the scene can be linked in some fashion  with a UFO sighting.
Behavior
According to the accounts of  those encountering them, Men in Black always seem to have detailed  information on the persons they contact, as if the individuals had been  under surveillance for a significant period of time. They have  occasionally been described as seeming confused by the nature of  everyday items such as pens, eating utensils or food, as well as using  outdated slang, vintage automobiles or outmoded styles of clothing.  Reports indicate that they often claim to be from an agency collecting  information on the unexplained phenomenon their subject has encountered.  In other accounts, they seem to be trying to suppress information by  trying to convince their target that the event in question didn't  happen. They have been described as behaving in either an exceedingly  furtive manner or a completely outgoing one, with wide grins and  disconcerting giggles. This observation might suggest that the  "gigglers" were possibly, though not necessarily, pranksters, as various  forms of unexpected, inappropriate, or even bizarre (as in robotic)  behavior seem to be hallmarks of these mysterious beings.