People are excellent at identifying faces familiar to them, even from very low quality images, but are bad at recognizing, or even matching, unfamiliar faces. In this review we shall consider some of the factors that affect our abilities to match unfamiliar faces. Major differences in orientation (e.g. inversion) or greyscale information (e.g. negation) affect face processing dramatically, and such effects suggest that representations derived from unfamiliar faces are based on relatively low-level image descriptions. Consistent with this, even relatively minor differences in lighting and viewpoint create problems for human face matching, leading to potentially important problems for the use of images from security videos. The relationships between different parts of the face (its 'configuration') are as important to the impression created of an upright face as the local features themselves, suggesting further constraints on the representations derived from faces. We go on to consider the contribution of computer face-recognition systems to the understanding of the theory and the practical problems of face identification. Finally, we look to the future of research in this area that will incorporate motion and 3-D shape information.