No two faces are alike, not even those of identical twins. Each face is unique. The human face is one of the most important social tools. An enormous variety of communication signals are produced using the face and it governs the expression of emotion, interest, desire and attention. We use our faces to attract, repel, scare, soothe and entertain. The face suggests details such as age, gender, culture, health and ethnic group. It is usually the first part of the body that we notice and the only part that we address. We are each capable of perceiving the smallest variation between faces and it is this ability that allows us to carry out personal recognition and identification. As adults we recognise and differentiate hundreds of faces of our family, friends, colleagues, famous people etc. This is illustrated by our ability to distinguish eventually the faces of identical twins (see Fig. 1.1). Even though the faces of identical twins are remarkably similar and they share the same genetic profile, their faces are in fact slightly different and people who know identical twins very well can distinguish them from one another.