  Molecular polaritons are the optical excitations which emerge when molecular
transitions interact strongly with confined electromagnetic fields. Increasing
interest in the hybrid molecular-photonic materials that host these excitations
stems from recent observations of their novel and tunable chemistry. Some of
the remarkable functionalities exhibited by polaritons include the ability to
induce long-range excitation energy transfer, enhance charge conductivity, and
inhibit or enhance chemical reactions. In this review, we explain the effective
theories of molecular polaritons which form a basis for the interpretation and
guidance of experiments at the strong coupling limit. The theoretical
discussion is illustrated with the analysis of innovative applications of
strongly coupled molecular-photonic systems to chemical phenomena of
fundamental importance to future technologies.
