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Abstract: Planar conductivity sensors are the subject of increasing interest as basic transducers for biosensors. The high degree of control of the performance characteristics undoubtedly forms an important argument in favour of conductivity-based sensing. The paper provides an outline of the design rules to be followed if an optimal design of a planar conductivity cell is required. Based on a simplified model, it is shown that the required accuracy establishes a lower limit to the overall sensor dimensions. This lower limit is expressed as a minimum longitudinal path length necessary to obtain the desired accuracy. Given an available area, the optimum ratio of electrode-width over inter-electrode spacing for a basic two-electrode structure is shown to be close to unity. Furthermore, it is shown that the decomposition of the two electrodes into an interdigitated structure decreases the accuracy of the device if all other parameters are considered constant. If the sensing region has to be limited to within a thin sensitive layer, the splitting is proposed of one of the electrodes into a compound electrode. The optimum lay-out of this compound structure is calculated as a function of the layer thickness.