Currently, systems for the detection of nucleic acid sequences, known as DNA-chips, are getting lots of attention. Such systems usually involve either an enzymatic or chemical labelling reaction as part of the detection process. The next generation of DNA-chips aims at a labelfree, fully electronic readout system. Several new approaches to signal generation that avoid a labelling step have been developed in recent years. Besides other surface sensitive measurements the possibility of electrochemical impedance and field-effect measurements for the detection of biomolecules have been discussed. The fully electronic detection of charged biomolecules based on the field-effect principle offers a labelfree method, which combines the unique sensitivity and selectivity of biomolecular recognition reactions with an electronic chip-based readout. In this approach one type of molecules is fixed at a surface and the biomolecular reaction with complementary molecules is detected by change in the drain-source current of the transistor. This change can occur by a change of the interface capacitance of the transistor gate or by change of the surface potential during adsorption of the molecules. At the moment a complete theoretical description of the detection principle is still under discussion. However, the fully electronic readout of biomolecular reactions offers a unique principle for the construction of many different sensors for bioassays. We are working on an approach to detect the hybridization of DNA sequences using electrolyte-oxide-semiconductor field-effect transistor (EOSFET) arrays. This method allows direct and in situ detection of specific DNA sequences without any labelling.