Resonant optical sensors typically rely on wavelength shifts (Δλ) of their spectral features, such as peaks in transmission and reflection, induced by analytes. To allow an accurate determination of the peak position and its quantitative dependence on the analyte concentration, one requires sensors with high quality factors (Q-factors), defined as the ratios of their resonance frequency by the corresponding linewidth. Since Q-factors are inversely related to the linewidth, which represents the losses of the resonant system, numerous strategies have been proposed to reduce these losses, and therefore sharpen the resonances and decrease the readout noise of optical sensors. These approaches include the use of low-loss materials and the tailoring of the resonator geometry as in nanoparticles-on-mirror, whispering-gallery-mode microcavities, and periodic metal nanoparticle arrays. In particular, periodic nanoparticle arrays achieve high Q-factors by two processes: first, they reduce the radiative losses of individual nanoparticles by destructive interference of the coherently scattered radiation by the nanoparticles in the array; second, they redistribute the electromagnetic field into the surroundings, thus outside the individual metallic nanoparticles where losses originate. This last condition benefits sensors probing phenomena occurring outside the metal nanoparticles, such as changes in the refractive index of the surrounding medium. On the other hand, the removal of the field from the metallic nanoparticles is unfavorable for other classes of plasmonic sensors that probe changes inside the metal; the so-called direct plasmonic sensors.