  Classically the interaction between light and matter is given by the Maxwell
relations. These are briefly reviewed and will be used as a basis to discuss
several techniques that are used in optical spectroscopy. We then discuss the
quantum mechanical description of the optical conductivity based on the Kubo
formalism. This is used as a basis to understand how strong correlation effects
can be observed using optical techniques. We will discuss the use of sum rules
in the interpretation of optical experiments. Finally, we describe the effect
of including interactions between electronic and collective degrees of freedom
on optical spectra.
