  When atoms come together and bond, we call these new states molecules, and
their properties determine many aspects of our daily life. Strangely enough, it
is conceivable for light and molecules to bond, creating new hybrid
light-matter states with far-reaching consequences for these strongly coupled
materials. Even stranger, there is no `real' light needed to obtain the
effects, it simply appears from the vacuum, creating `something from nothing'.
Surprisingly, the setup required to create these materials has become
moderately straightforward. In its simplest form, one only needs to put a
strongly absorbing material at the appropriate place between two mirrors, and
quantum magic can appear. Only recently has it been discovered that strong
coupling can affect a host of significant effects at a material and molecular
level, which were thought to be independent of the `light' environment: phase
transitions, conductivity, chemical reactions, etc. This review addresses the
fundamentals of this opportunity: the quantum mechanical foundations, the
relevant plasmonic and photonic structures, and a description of the various
applications, connecting materials chemistry with quantum information,
nonlinear optics and chemical reactivity. Ultimately, revealing the interplay
between light and matter in this new regime opens attractive avenues for many
applications in the material, chemical, quantum mechanical and biological
realms.
