Classifying selectors are used to control the population of foam causing organisms in activated sludge plants to present the development of nuisance foams. The term “classifying selector” refers to the physical mechanism by which it selects against these organisms; foam causing organisms are enriched into the solids in the foam and their rapid removal controls their population to low levels in the mixed liquor. Foam causing organisms are wasted “first” rather than the usual case where they accumulate on the surface of tanks and thereby are wasted “last.” The first US application was at full-scale within a sludge reaeration tank at an Atlanta plant and achieved with excellent results. Brown and Caldwell has since applied classifying selectors at seven plants. The design concepts, retrofit approaches and operating experience are reviewed.