Collective excitations in matter, either in the form of plasmons in metals or Mie resonances in high-index dielectrics, constitute fundamental building blocks for photonic applications, including optical and quantum communications, biosensing, and polaritonic chemistry. Motivated by these prospects, this book explores excitations sustained in metallic and dielectric nanocavities and their interaction with light, quantum emitters, and swift electrons. The book presents analytic methods for describing the strong and weak coupling between spherical nanocavities and optical transitions in matter, while also analyzing the impact of nonlocality on the coupling, when considering extremely small metallic structures. Besides light-based spectroscopy, it examines electrons as sources for probing optical excitations in matter and provides analytic tools for simulating measurements in cathodoluminescence and electron energy-loss spectroscopy.