Publisher Summary This chapter studies the semiclassical models of infrared intensities. The theoretical analysis of the observed wavenumbers of vibrational transitions is aimed primarily at determining the potential force field of molecules and the form of vibrations as reflected in the elements of the normal coordinate transformation matrix. The intramolecular binding forces are quantitatively represented through the force constants matrix. It is now recognized that the physically most plausible way of expressing intramolecular forces is by defining the matrix coefficients as second derivatives of the potential energy with respect to internal vibrational coordinates. The intensities in infrared spectra are determined by the intramolecular charge shifts accompanying the normal vibrations of molecules. The measured band envelopes contain quite essential information about the distribution and dynamics of electric charges in molecules. An important step toward reducing the experimental absorption intensities into molecular intensity parameters is the evaluation of the matrix. It is found that symmetric properties of molecular vibrations are introduced in a natural way with all consequent simplifications.