This chapter presents the application of planar microwave sensors based on coupled resonators for the detection of solute concentration in aqueous solutions. First, the dielectric and conductive properties of these solutions, based on broadband experimental measurements, are presented. Then, different aspects of the sensor design, as well as a sensor circuit model, are exposed. For this model, the theoretical response and the influence of the mutual and self-capacitances of the resonators on the sensitivity are studied. The last section presents two practical examples of sensors applied, respectively, to binary (water-glucose) and ternary (water-sucrose-sodium chloride) solutions.