The opacity mechanism is the only well understood excitation mechanism of observed stellar oscillations. The driving effect arises due the a specific form of the opacity perturbation which leads to a conversion of the radiative flux energy into kinetic energy of oscillations. In 1963 Baker and Kippenhan demonstrated that the opacity mechanism acting in the Hell ionization zone is the cause of the Cepheid pulsations. Subsequently it has been shown that the same mechanism works in a large variety of stars. There are three opacity bumps that play a role in pulsation excitation in various types of objects - one connected with the H ionization, one connected with He II ionization zone - and one, discovered quite recently by Iglesias, Rogers & Wilson (1988), caused by a large agglomeration of metal element (mostly Fe) lines.