In this book the sensing properties of plasmonic resonators for changes in the surrounding refractive index are investigated. A self-consistent and general sensing theory is developed. This theory connects the electrodynamic properties of plasmonic resonators like resonance wavelength and electric field distribution to the sensitivity for refractive index changes. A figure of merit (FOM) is derived which includes the effects of noise and in its general form directly states if a certain change in refractive index will be measurable or not. For the FOM in the quasi-static limit absolute bounds and scalings are derived. These bounds are based on the localization of electromagnetic energy for which analytic expressions were known before. To confirm the developed theory numerical calculations and an experiment with crescent shaped plasmonic resonators is carried out and good agreement is found.