New generation biosensors are analytical compact devices made of thin films. Sensitivity, specificity, rapid response time, ease-of-use, and low cost are the major advantages of these biosensors. All of these properties are closely related with thicknesses of the films used in fabrication of the sensor. The detection principle of a biosensor is mainly based on the interaction of the biological analyte with the surface-modified thin film. The thin film acts as a physicochemicaloptical, mechanical, magnetic, and electricaltransducer and converts the signal resulting from the recognition of the biological analyte into another measurable signal. In this chapter, first, the roles of thin films in biosensor applications will be discussed. Then, different types of thin films used in the fabrication of biosensors will be explained. The methods to form organic thin films on sensitive layers for adsorption of biological analytes will be given together with four main methods of detection as: optical, mechanical, magnetic, and electrical. Finally, recent developments will be outlined.
