Recent progress in laser technology makes it possible to detect the spectral broadening of light scattered from solutions of macromolecules. Since the spectral width of light from a CW gas laser, e. g., a He-Ne laser, is only a few Hz or less, we can detect a frequency shift as small as 10 Hz if we adopt a photoelectric lightmixing technique for the detection system. In this review article, a newly developed technique, quasielastic light scattering, will be presented. The theory of the spectrum of laser light scattered from solutions of macromolecules is at first reviewed. The experimental apparatus required to measure the spectrum is then described. Finally, the application of the technique to the elucidation of problems of biophysical interest is discussed.