This thesis deals with some recent advances in nanophotonics and, in particular, with the rapid steps it has taken to approach quantum optics and quantum physics in general. It summarises my theoretical work —either done independently or in collaboration with experimentalists— in two areas with different starting points but converging routes: those of quantum-informed plasmonics and strong coupling. Most of the work presented here can be classified in one or the other area, while I also include some of my recent research on effects that can be seen as precursors of these areas, and my attempts to unify the two, predicting to some extent future experimental and theoretical developments. The thesis starts with a detailed introduction to the field of nanophotonics (Chapter 1), where I first discuss its development, giving some historical perspective, and then focus on recent trends and advances. I show, in particular, how nanophotonics has been an experiment-driven field, where new developments have mostly originated from advances in nanofabrication and characterisation techniques that extend the application of known concepts to gradually smaller dimensions. Nevertheless, this continuous downscaling has now reached the point where concepts and methods of classical physics (as in classical optics) have been rendered insufficient, and theoretical descriptions inspired from quantum mechanics or extending its use to the mesoscopic world in a realistic and computationally efficient manner are required. Chapter 2 presents the theory required to follow the activities and results analysed in subsequent chapters. The chapter starts with a review of the main elements of classical electrodynamics (CED) and how it applies to nanoscopic objects excited by plane electromagnetic (EM) waves, point dipoles, or electron beams. It introduces the concepts of surface plasmon polaritons (SPPs) in metal films, localised surface plasmons (LSPs) in metallic nanoparticles (NPs), and Mie resonances in their dielectric counterparts. Since most of the deviations from an accurate description within CED originate not from insufficiencies in Maxwell’s equations for CED —which still constitute one of the most beautiful and complete theories in physics— but from a poor description of the material response and the boundary conditions, elements of solid-state physics regarding materials and their dielectric functions are then discussed, and their modification within quantum-informed models is described. Finally, the chapter ends with a description of the computational methods employed in this thesis. In Chapter 3 the methods described in Chapter 2 are applied to explore the optical response of single metallic NPs or interacting NPs in aggregates, to establish the role of quantum-informed models in plasmonics, as long as the far-field properties of the system are concerned. The chapter focuses on two nonlocal hydrodynamic-based models, the hydrodynamic Drude model (HDM), which efficiently accounts for electron screening, and the generalised nonlocal optical response (GNOR) theory, which offers an elegant and straightforward way to introduce surface-enabled Landau damping. An application of the Feibelman d-parameter formalism is also included, serving as a hint of what future research might concern. To compare with other existing models, an example where the quantum-corrected model (QCM) for tunnelling is used is also included. This example is actually proposed as an efficient way to disentangle quantum-informed models for plasmonics, and assess their validity, limitations, and range of applicability. Finally, the far-field optical response of dielectric NPs is also discussed here, so that the remaining chapters can focus on the interaction of such photonic nanostructures with classical or quantum emitters. Chapter 4 deals with the weak coupling of quantum emitters to plasmonic nanostructures with ultrasmall geometric details, where quantum effects are anticipated to play a role in the relaxation times of the system. Electric point dipoles modelling two-level systems are placed in the vicinity of metallic NPs or NP aggregates, and the influence of quantum corrections on the well-known fluorescence enhancement of such systems is explored, focusing on length scales and materials for which the HDM/GNOR models are known to accurately predict the far-field optical response. While the thesis contains no publication in which dielectric NPs are considered as a template for weak-coupling studies, the chapter ends with one such calculation for completeness. Finally, in Chapter 5 quantum-informed models and quantum optics are combined, to explore the strong coupling of emitters with tailored nanophotonic environments acting as (open, poor) cavities in analogy with cavity quantum electrodynamics (cQED). The first part of the chapter explores how quantum-informed models affect the formation of plasmon–exciton hybrids (plexcitons) in plasmonic cavities, excited either optically or by high-energy electron beams, while the second part introduces high-index dielectric NPs as efficient alternatives, to excite novel hybrid Mie-exciton polaritons, and addresses briefly the possibility to externally control them with a static magnetic field. An interlude, which to a certain extent dominates the spirit of the chapter, is based on a recent review paper (Publication N) which critically discusses the transfer of concepts from QED to nanophotonics, a tactic in which one needs to be particularly careful. A summary of the main results is presented in Chapter 6, placing the thesis in its appropriate context compared to recent international activities. This has also been done throughout the thesis, with a —I think— quite extensive reference list. This summary is followed by a description of my own ongoing and future work. By this point, I hope that the reader will have been convinced that the presented work, while naturally just a small part of what the community has invested in, is indeed interesting, timely, competitive and innovative, and constitutes just a few steps in what promises to be a long journey.