Basic aspects and examples of biosensors, including definitions and applications, response of enzyme-based biosensors, examples of biosensor configurations, ferrocene-mediated amperometric glucose sensor, potentiometric biosensor for phenyl acetate, potentiometric immunosensor for digoxin, evanescent-wave fluorescence biosensor for bungarotoxin, optical biosensor for glucose based on fluorescence energy transfer, piezoelectric biosensor for nucleic acid detection, enzyme thermistors, evaluation of biosensor performance.