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 concept originated in South Africa where it was shown through pilot studies that placement of a flotation tank for foam removal prior to secondary clarifiers would eliminate foam causing organisms from the process. A variant of the concept called “selective foam wasting” was implemented at full-scale within a sludge reaeration tank at an Atlanta plant with excellent results; it was operated on a campaign basis when foam reached nuisance levels. Brown and Caldwell modified the concept to normally waste on a continuous basis and has since applied classifying selectors at seven plants. The design concepts, retrofit approaches and operating experience are reviewed.