  Atomic vibrations in perfect, slightly defective or mixed crystals are to a
large extent well understood since many decades. Theoretical descriptions are
thus in excellent agreement with the experiments. As a consequence,
phonon-related properties like specific heat, thermal conductivity or sound
attenuation are also well explained in these solids. This is not yet the case
in glasses where the lack of periodicity generates enormous difficulties in
theoretical treatments as well as in experiments or in numerical simulations.
Thanks to recent developments along all these lines, comprehensive studies have
emerged in the last decades and several decisive advances have been made. This
chapter is thus devoted to a discussion of the nature of the vibrational
properties in glasses with particular emphasis on the low-frequency part of the
vibrational density of states, including the acoustic excitations, and of the
experimental techniques used to their study.
