In this tutorial, I will detail how the quantum or dielectric confinement of electronic wavefunctions can be exploited to achieve optical resonances at the nanoscale. I will then explain how classical electrodynamic theory can be effectively used to study the coupling between quantum emitters or between dielectric resonators but also between quantum emitters and dielectric resonators in both the weak and strong coupling regimes.

I will discuss how this coupling can be used to design complex nanoscale resonators featuring enhanced optical near fields but also to produce the molecular aggregates used in light-harvesting systems. Furthermore, I will show how the formulation of the Purcell factor can be used to analyze luminescence enhancement effects but also to characterize non-radiative energy transfer between fluorescent molecules, highlighting the applications of these phenomena in sensing and biophysics. I will also detail the stringent experimental conditions required to reach a strong coupling regime between quantum emitters and dielectric resonators at room temperature and show some recent realizations, opening exciting perspectives for quantum technologies operating close to room temperature.