In this paper, biosensors are the device in which there is a coupling of biological sensing element whit a detector system using a transducer. In comparison with any other currently available diagnostic device, biosensors are much higher in performance in terms of sensitivity and selectivity both. Biosensors have found potential application in the industrial processing and monitoring, environment pollution control, also in agriculture and food industries. Important features for commercialization of the biosensors are selectivity, sensitivity, stability, reproducibility and low cost. This article reviews the brief history, basic principle, and the various types of biosensors available.