During recent years in chemical engineering processes for instance with aerated stirred tanks containing surface-active components foams and their mechanical control have become very important. However, there are still no rules for calculating mechanical foam breakers. As a contribution to foam control the present paper discusses the fundamentals of foam breaking by means of foam densities and gives a physical model for the mechanisms of a mechanical foam breaker when the main breaking effect is based on impact. The mechanisms were proved by measuring pressures in a foam breaker box. Diameters of secondary foam bubbles produced by foam breakers are calculated and discussed. Experimental investigations were made with different surfactants in aqueous solutions. The paper points out their influence on the process of foam breaking.