An amperometric sensor is a detection and quantitative measurement device in which redox events associated with the selective recognition of an analyte of interest are processed by an electrochemical transducer to produce signal in the form of current, the magnitude of which is proportional to the quantity of analyte present. The recognition component of an amperometric sensor usually consists of an electroactive redox substance or composite of chemical or biological origin intimately connected to the transducer's electronics. This entry deals with a special type of amperometric sensors referred to as biosensors. This class of sensors incorporates at least one biochemical step in the analyte recognition process which is coupled to the interfacial electron transfer reaction at the electrode, either directly or through an electron transfer mediator. Biomolecules (particularly redox enzymes), their immobilisation or electron relay platforms, and transducer (electrode) formats used in the development of amperometric sensors are described. Also presented in this entry are electrochemical methods used in biosensor metrology, sensor operational and performance indices, the effect of solvents in biosensing process, kinetic modelling of the heterogeneous electrocatalytic reaction of sensors, and the applications of biosensor technology.