After 9/11 tragedy, governments in all over the world started to look more seriously to the levels of security they have at their airports and borders. Countries annual budgets were increased drastically to have the most recent technologies in identification, recognition and tracking of suspects. The demand growth on these applications helped researchers to be able to fund their research projects. One of most common biometric recognition techniques is face recognition. Although face recognition is not as accurate as the other recognition methods using iris or fingerprints, it still grabs huge attention of many researchers in the field of computer vision. The main reason behind this attention is the fact that the face is the conventional and the fastest way people use to identify each other. Since the mid of last century the researchers have been working extensively in the field of face recognition trying to introduce new approaches or improve the performance of the available approaches to obtain robust and accurate face recognition systems.