Dispersion relations for optical properties of solids, and a sum rule for the imaginary part of the dielectric constant, are reviewed and applications to semiconductors are described. Plasma frequencies, for which the real part of the dielectric constant vanishes, are associated with lattice vibrations, with free carriers, and with valance electrons. Gottlieb's infrared optical constants for LiF are in good agreement with the lattice vibration sum rule using unit effective charge. Dispersion relations for reflectivity have been used by Dixon to analyze optical constants ofp‐type PbTe in the infrared region where free‐carrier effects dominate. He finds an effective mass at room temperature which rises from 0.1mto 0.3mas the carrier concentration is increased from 3×1018to 1020cm−3. Morrison has measured the reflectivity of InAs, InSb, and GaAs and finds curves which agree well with those of Tauc and Abraha´m. His analysis of these results using the dispersion relation predicts a plasma energy near 7 ev in these materials. Free carrier absorption in several III–V compounds and in PbS and PbTe is proportional to &lgr;pat long wavelengths, wherepvaries between 2, for InSb and AlSb, and 3, for InAs and GaAs. Forn‐ andp‐type PbS Riedl findsp=2.4. He observes additional structure near the intrinsic absorption edge ofp‐type PbTe, resembling that observed inn‐type GaAs and GaP, which may be associated with a valence band about 0.1 ev below the band edge. The absorption coefficient near the absorption edge of all the lead compounds and of InAs is proportional toeℏ&ohgr;/kTeff, with 80°K<Teff<120°K for measurements taken at room temperature. Impurity effects on the energy gap are discussed in terms of a simple model.