  Investigating electronic structure and excitations under extreme conditions
gives access to a rich variety of phenomena. High pressure typically induces
behavior such as magnetic collapse and the insulator-metal transition in 3d
transition metals compounds, valence fluctuations or Kondo-like characteristics
in $f$-electron systems, and coordination and bonding changes in molecular
solids and glasses. This article reviews research concerning electronic
excitations in materials under extreme conditions using inelastic x-ray
scattering (IXS). IXS is a spectroscopic probe of choice for this study because
of its chemical and orbital selectivity and the richness of information it
provides. Being an all-photon technique, IXS has a penetration depth compatible
with high pressure requirements. Electronic transitions under pressure in 3d
transition metals compounds and $f$-electron systems, most of them strongly
correlated, are reviewed. Implications for geophysics are mentioned. Since the
incident X-ray energy can easily be tuned to absorption edges, resonant IXS,
often employed, is discussed at length. Finally studies involving local
structure changes and electronic transitions under pressure in materials
containing light elements are briefly reviewed.
