Do the MIB exist? Who are these MIB and where do they come from? There are a number of theories regarding their origin, yet, one thing is for certain - their strange, menacing, salesman-like appearance. They often appear immediately following a UFO close encounter. They drive large, all black, old model American cars. They are often described as having olive complexions, thin lips, and strange eyes. Witnesses recall these MIB as having monotone speech and the very odd behaviors they display while taking UFO evidence (photos, recordings etc..) from their homes. They are always in black, hence the name, and in a group of 2 or 3. Most UFO witnesses never get visited by MIB, but for those that do the main goal is often met - MIB use crafty scare tactics to silence witnesses.
Source: Men in Black Facts - UFO Evidence
Of course, there are many skeptics who say these MIB do not exist and these sightings are all fictitious. Gray Barker was a well-known, American UFO conspiracy writer who wrote a book called "They Knew Too Much" where the idea of MIB is said to have originated. People closest to Barker claim he fabricated UFO related stories and embellished upon cases involving the government for financial gain. See: Shades of Gray .
Interested in some reading material on the subject? Check these books out.
The Real Men In Black: Evidence, Famous Cases, and True Stories of These Mysterious Men and their Connection to UFO Phenomena
Mystery of the Men in Black: The UFO Silencers