Competitive Exclusion (CE) describes the protective effect of the intestinal microflora against colonization of pathogenic bacteria. This review presents the background and current use of the CE concept to prevent Salmonella infections in poultry, by administration of cultures of intestinal origin to day-old chickens. This approach is a well-documented tool for effective mitigation of the risk for Salmonella infections in newly hatched chickens from being further spread horizontally in a flock or vertically in case of breeding animals. The administrated cultures compensate for the delayed development of the protective intestinal microflora in chickens hatched under hygienic conditions in modern poultry production.