Foam stability is closely related to the liquid’s surface viscosity under non-equilibrium unstable conditions. Surface viscosity measurements for surfactants, lipids, or protein solutions give reliable indices for the foam stability such as when stable soap bubbles can be made by adding thickening agents to the soap solution. There are mainly three methods to measure surface viscosity of aqueous solutions with small amounts of surface active materials such as surfactants or proteins: the damped oscillation, rotation, and capillary methods. In this chapter, damped oscillation is focused on as it is the most common method. Foaming is a key process for industrial products such as foamed plastics and cement, while anti-foaming is also important for the engineering process of food, fermentation, textiles, dyes, paints, ink, and petrochemical production.