Experiences of numerous full-scale biogas plants on spontaneous foam formation were evaluated. Focus was laid on the feedstock mix, the causes of foaming, and control strategies. Mostly, the main digester was affected, but excessive foam formation in the post digester was found as well. The feedstock mix was stated as the main cause of foam formation followed by process troubles, abrupt degassing, temperature changes, and changes in viscosity. In the feedstock of plants affected by foaming, manure, cereals, sugar beets, and cosubstrates were used most often. Successful control strategies were gathered and categorized into seven groups: emergency measures, application of antifoam additives, avoidance of foam-forming substrates, prevention of process upsets, changes of physicochemical conditions, optimization of mixing, and installation of technical equipment against foam formation.