  Physical and chemical systems can be characterized by their natural frequency
and energy scales. It is hardly an exaggeration that most of what we know about
such systems, from the acoustics of a violin to the energy levels of atoms,
comes from their response to perturbations at these natural frequencies. It is
of course the same situation in `correlated' electron materials. We can learn
about the novel effects of strong electron-electron interactions and the
properties of collective states of matter (superconductors, quantum magnets
etc.) by characterizing their response to small amplitude perturbations at
their natural frequencies. In solids, these natural frequency scales span an
impressively large frequency range from x-ray down to DC. This incredibly broad
range means that a blizzard of experimental techniques and analysis methods are
required for the characterization of correlated systems with optical
techniques. This short review and lecture notes attempt to lay out a brief
summary of the formalism, techniques, and analysis used for `optical'
spectroscopies of correlated electron systems. They are idiosyncratic,
occasionally opinionated, and - considering the breadth of the subject -
incredibly brief.
