Biometric recognition involves methods of automatically recognizing people by their biological and/or behavioral characteristics. Biometric systems have now been deployed in various physical and logical access control applications as well as in forensic applications. This paper describes the architecture and the basic modules of a biometric recognition system, as well as the information flow during the enrollment and recognition (verification and identification) phases of the system operation. Different rates for measuring system performance are introduced. The unimodal systems are discussed and the reasons for introducing multimodal systems, based on integration schemes that fuse the information obtained from sensor to decision level, are presented. Finally, some concluding remarks, open problems and limitations of biometric recognition systems are given.