  The exchange of energy between electronic and nuclear motion is the origin of
non-adiabaticity and plays an important role in many molecular phenomena and
processes. Conical intersections (CIs) of different electronic potential energy
surfaces lead to the most singular non-adiabaticity and have been intensely
investigated. The coupling of light and matter induces conical intersections
which are termed light-induced conical intersections (LICIs). There are two
kinds of LICIs, those induced by classical (laser) light and those by quantum
light like that provided by a cavity. The present work reviews the subject of
LICIs, discussing the achievements made so far. Particular attention is paid to
comparing classical and quantum LICIs, their similarities and differences and
their relationship to naturally occurring CIs. In contrast to natural CIs, the
properties of which are dictated by nature, the properties of their
light-induced counterparts are controllable by choosing the frequency and
intensity (or coupling to the cavity) of the external light source. This opens
the door to inducing and manipulating various kinds of non-adiabatic effects.
Several examples of diatomic and polyatomic molecules are presented covering
both dynamics and spectroscopy. The computational methods employed are
discussed as well. To our opinion, the young field of LICIs and their impact
show much future potential.
