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Omnipresent impurities such as carbon, oxygen, silicon, and hydrogen play important roles, both detrimental and beneficial, in the fabrication of solid-state devices. The electronic and vibrational properties of semiconductors are significantly altered by the presence of impurities. Atoms that are less massive than the host atoms, typically, show local vibrational modes (LVMs). Unlike lattice phonons, LVMs are localized in both the real and frequency domains, giving rise to sharp peaks in infrared-absorption and Raman-scattering spectra. The isotopic composition of the impurity and the surrounding atoms results in well-defined shifts in the vibrational frequencies. In Ge, GaAs, and CdTe, the host–isotope disorder leads to complex vibrational spectra that can be simulated by empirical, quasimolecular models. External parameters such as temperature and pressure (uniaxial and hydrostatic) have been tuned over a wide range to yield information about symmetry and impurity–host interactions. In this article, issues relating to LVMs in semiconductors are reviewed, with an emphasis on infrared and Raman spectroscopy. Experimental and theoretical studies of LVMs in semiconductors are described for several representative examples. © 2000 American Institute of Physics.