Studies on the basic properties and fabrication of biosensors based on conducting organic polymers have been reviewed. Several types of biosensors and some classes of conducting polymers used in biosensors are described. It is shown that their selective responses towards a dissolved analyte have lead to their successful application in a new generation of ion, molecular and biosensors. Some important open problems in this fast growing field have also been indicated. They are related with the complex interactions of biomolecules and/or cells with the conducting transducers. The present tendency to extreme miniaturization of biosensors related to the utilization of a few biomolecules and/or a single cell as a heart of biosensing devices seems to be very close to molecular electronics