Popularized by the former Fox drama Lie to Me, the micro-expression is an actual phenomenon. In the show, the character Cal Lightman, played by Tim Roth, used his keen eye to read people's micro-expressions to see if they were lying. Although some aspects of the show seemed quite farfetched the main points were truthful. Micro-expressions are brief, involuntary facial expressions according to emotions experienced. Although this definition sounds quite similar to the definition to a normal facial expression, the key word is involuntary. These expressions are next to impossible to fake.
Paul Ekman, a leader in the field of facial expressions, defied the long standing belief that expressions are cultural. His theory states that there are eleven universal expressions that all humans display in similar ways. They are as follows: amusement, contempt, contentment, embarrassment, excitement, guilt, pride, relief, satisfaction, pleasure, and shame. Each of theses expressions has a specific corresponding micro expression.
These methods of analyzing facial expressions have been used by many government agencies including the Transportation Security Administration. The Screening Passengers by Observational Techniques (SPOT) program has been implemented at over 161 airports in the USA alone. Although alot of training is required to be come licensed SPOT officer, there have still been quite a few cases of blatant officer profiling as opposed to science based analysis. This has raised some concerns that the methods of Dr. Ekman are pseudoscience not fact based science. Ekman, of course, has defended his methods after all of the accusations of skeptics. Simply put his opinion is not the science that is at fault, but the person is looking for the wrong cues due to poor training and laziness.
Article about Ekman and the TSA
Further reading on Ekman's work and implementation in the real world