The monograph of the American physicist M. Cardona, one of the leading experts in the field of optical research, is devoted to the latest methods of studying the optical spectra of solids, based on the removal of differential curves by modulating a parameter (external electric field, wavelength, pressure, temperature, etc.). The modulation technique described in detail by the author allows several orders of magnitude to increase the sensitivity and accuracy of measurements compared to conventional methods. Much space is also devoted to comparing the experimental data given with the results of theoretical calculations.