Most information on the width of phonons in crystals has been obtained by laser Raman spectroscopy: The resolution usually available in INS, and more recently in Inelastic X-ray Scattering, is too poor to yield precise results on phonon widths. Recently, phonon width data have also been obtained by coherent phonon techniques (in the time domain) and by neutron echo experiments, but these methods apply only to phonons of relatively low frequency. Theoretically, the phonon widths are described as the imaginary part of a self-energy, its real part representing a phonon shift beyond the harmonic approximation. The self energy is temperature dependent, a fact that accounts for an increase in width with increasing temperature. The shift  usually corresponds to a softening with increasing temperature, although the opposite is sometimes found. The mechanisms involved in this self- energy are either anharmonic interactions (e.g. decay into two phonons) and electron-phonon interactions (in the case of metals and semiconductors).

Several typical and also unusual experimental results involving semiconductors and high Tc superconductors will be discussed together with their theoretical interpretation on the bases mantioned above.

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