  Infrared spectroscopy has emerged as a premier experimental technique to
probe enigmatic effects arising from strong correlations in solids. Here we
report on recent advances in this area focusing on common patterns in
correlated electron systems including transition metal oxides, intermetallics
and organic conductors. All these materials are highly conducting substances
but their electrodynamic response is profoundly different from the canonical
Drude behavior observed in simple metals. These unconventional properties can
be attributed in several cases to the formation of spin and/or charge ordered
states, zero temperature phase transitions and strong coupling to bosonic
modes.
