  This is a tutorial-style introduction to the field of molecular polaritons.
We describe the basic physical principles and consequences of strong
light-matter coupling common to molecular ensembles embedded in UV-visible or
infrared cavities. Using a microscopic quantum electrodynamics formulation, we
discuss the competition between the collective cooperative dipolar response of
a molecular ensemble and local dynamical processes that molecules typically
undergo, including chemical reactions. We highlight some of the observable
consequences of this competition between local and collective effects in linear
transmission spectroscopy, including the formal equivalence between quantum
mechanical theory and the classical transfer matrix method, under specific
conditions of molecular density and indistinguishability. We also overview
recent experimental and theoretical developments on strong and ultrastrong
coupling with electronic and vibrational transitions, with a special focus on
cavity-modified chemistry and infrared spectroscopy under vibrational strong
coupling. We finally suggest several opportunities for further studies that may
lead to novel applications in chemical and electromagnetic sensing, energy
conversion, optoelectronics, quantum control and quantum technology.
