In this paper, we provide an overview of the fundamentals of biometric identification, together with a description of the main biometric technologies currently in use, all of them within a common reference framework. A comparison on different qualitative parameters of these technologies is also given, so that the reader may have a clear perspective of advantages and disadvantages of each. A section on multibiometrics describes the state of the art in making these systems work coordinately. Fusion at different conceptual levels is described. Finally, a section on commercial issues provides the reader a perspective of the main companies currently involved in this field.