Proteins are very surface‐active substances, which means that they readily adsorb onto O/W and A/W interfaces, even if present in very low concentration. They do not yield a very low interfacial tension. Adsorption, conformation in the interface, and the various interfacial properties resulting from their adsorption are briefly reviewed, especially in relation to formation and stability of emulsions and foams. Displacement from the interface by small‐molecule surfactants is also discussed. It is concluded that most effects of adsorbed proteins on the various kinds of physical instability of emulsions and foams are semiquantitatively understood, but that the variation among proteins in the production of small droplets and bubbles during emulsification and foaming, respectively, still largely defies explanation. It makes little sense to classify proteins according to their suitability for emulsification or foaming, because each of the physical properties of these systems may depend in a different way on protein species and on physicochemical conditions.