This paper covers the field of biometric systems with a special focus on biometric authentication systems. A short and general overview of biometric authentication systems gives some insight in how the various biometric data can be used for authentication. After that an abstracted view of biometric systems is given, followed by general performance evaluation techniques. The problems of varying biometric data, caused by noise respectively human nature and approaches to solve these problems with multi-biomteric systems in combination with information fusion, are also discussed. Since there is such a vast range of possibilities for the usage of biometric systems, some type of open system standardization is necessary.