Transient grating experiments in glass-former liquids

The transient grating experiment is a very useful spectroscopic tool to investigate glass-former and viscous liquid dynamics. The technical improvements, intro- duced in the recent years, transformed it in a unique spectroscopic technique, thanks to the wide time window covered and the quality of experimental data. Furthermore, only recently a precise definition of the measured response func- tion has been obtained permitting a more exact description of several dynamical processes present in complex liquids. In this chapter we review the transient grat- ing spectroscopy from both experimental and theoretical points of view, paying particular attention to the interpretation of the measured data.