  The physics of systems that cannot be described by a Hermitian Hamiltonian,
has been attracting a great deal of attention in recent years, motivated by
their nontrivial responses and by a plethora of applications for sensing,
lasing, energy transfer/harvesting, topology and quantum networks.
Electromagnetics is an inherently non-Hermitian research area because all
materials are lossy, loss and gain distributions can be controlled with various
mechanisms, and the underlying systems are open to radiation. Therefore, the
recent developments in non-Hermitian physics offer exciting opportunities for a
broad range of basic research and engineering applications relevant to the
antennas and propagation community. In this work, we offer a tutorial geared at
introducing the unusual electromagnetic phenomena emerging in non-Hermitian
systems, with particular emphasis on a sub-class of these systems that obey
parity-time (PT) symmetry. We discuss the basic concepts behind this topic and
explore their implications for various phenomena. We first discuss the basic
features of P, T and PT operators applied to electromagnetic and quantum
mechanical phenomena. We then discuss the exotic response of PT-symmetric
electromagnetic structures and their opportunities, with particular attention
to singularities, known as exceptional points, emerging in these systems, and
their unusual scattering response.
