Brillouin spectroscopy (BS), or Brillouin light scattering spectroscopy, is a very useful tool for studying mechanical properties of polymer nanocomposites. After a general presentation of the BS technique, with its theoretical basis, some technical considerations and the main scattering geometries used, some applications to polymers and polymer nanocomposites will be presented in this chapter. This chapter gives indications about what the important parameters for the sample are, and which should be considered by a reader aiming to perform a BS experiment: such as opacity, dimension, homogeneity, etc. The acoustic wave velocities, the elastic constants, the refractive index, various phase transitions, and in particular the glass transition temperature of polymers and nanocomposites are investigated from Brillouin scattering measurements. The variation of the storage modulus with the nanoparticle concentration is also explored. The fruitful coupling of Brillouin spectroscopy with other techniques is done with two examples. Brillouin and Raman spectroscopies monitor the in situ polymerization and the cross-linking processes during the network formation of epoxy, whereas the Brillouin scattering and friction experiment of nanocomposite correlate elastic modulus and tribological properties. Some references about the studies of biological systems are also provided.