Light can be localized to extremely small regions, down to subnanometer dimensions, by metallic nanoresonators that support resonant oscillations of the free electron cloud called plasmonic resonances. The extreme localization induced by the plasmonic (or related) excitations introduces new possibilities for fundamental studies and applications, including the mapping of the optical properties of molecules with submolecular resolution, the excitation of conventionally forbidden transitions or the optimization of the coupling with different matter excitations. The traditional treatment of plasmonic resonances based on solving the classical Maxwell’s equations can fail in these extreme conditions, so that a quantum description can become necessary. Here, we present and overview of novel effects in nanophotonic systems that are revealed by the use of different quantum methodologies, paying special to plasmonic nanoresonators characterized by extreme field localization and their interaction with molecules or other quantum emitters. We show that a rich variety of effects in molecular spectroscopy and microscopy emerge that are within reach of current experimental capabilities Selected references [1] J. J. Baumberg et al. Nano Lett 23, 10696–10702 (2023) [2] A. Babaze et al. Nanophotonics 12, 3277-3289 (2023) [3] R. Esteban et al. Acc. Chem. Res. 55, 1889−1899 (2022) [4] A. Babaze et al. Nano Lett. 21, 8466–8473 (2021). [5] T. Neuman et al. Nano Lett. 18, 2358−2364 (2018) [6] Y. Zhang et al. Nature Commun. 8, 15225 (2017) [7] W. Zhu et al. Nature Communications 7, 11495 (2016) | 
