  Polaritonic chemistry exploits strong light-matter coupling between molecules
and confined electromagnetic field modes to enable new chemical reactivities.
In systems displaying this functionality, the choice of the cavity determines
both the confinement of the electromagnetic field and the number of molecules
that are involved in the process, whereas in wavelength-scale optical cavities
light-matter interaction is ruled by collective effects, plasmonic
subwavelength nanocavities allow even single molecules to reach strong
coupling. Due to these very distinct situations, a multiscale theoretical
toolbox is then required to explore the rich phenomenology of polaritonic
chemistry. Within this framework, each component of the system (molecules and
electromagnetic modes) needs to be treated in sufficient detail to obtain
reliable results. Starting from the very general aspects of light-molecule
interactions in typical experimental setups, we underline the basic concepts
that should be taken into account when operating in this new area of research.
Building on these considerations, we then provide a map of the theoretical
tools already available to tackle chemical applications of molecular polaritons
at different scales. Throughout the discussion, we draw attention to both the
successes and the challenges still ahead in the theoretical description of
polaritonic chemistry.
