A Biosensor consists of two components: a bioreactor and a transducer. The bioreceptor is a biomolecule that recognizes the target analyte and the transducer converts the recognition event into a measurable signal. The uniqueness of a biosensor is that the two components are integrated into one single sensor. This combination enables one to measure the target analyte without using reagents. For example, the glucose concentration in a blood sample can be measured directly by a biosensor by simply dipping the sensor in the sample. The simplicity and the speed of measurement are the main advantages of a biosensor. In this module, fundamental concepts of analysis, design and fabrication of common biosensors are included with a special emphasis on glucose biosensor.