A properly functioning milking machine system requires a vacuum controller that maintains a nearly uniform vacuum level from no-load to full-load operation. Experiences with commercially available equipment on California dairies indicate that some types of controllers perform markedly better than other types. In this analysis of vacuum controller performance, controller terminology is redefined, a method for field testing controller performance is described, and six vacuum controllers in common use are rated as to how they meet the operating requirements of a good milking machine installation. Performance tests showed considerable variation in their sensitivity, and the need for better engineered controllers is suggested.