Probiotics can be consumed in various forms of fermented or non-fermented food products. As a common feature, probiotics become, after ingestion, transient constituents of the intestinal microbiota capable of exerting their biologic effects, thus giving a rationale for their use as component of functional foods. Weaning, stress, dietary changes, use of antibiotics, and intestinal infections are all conditions that affect the natural balance of intestinal microbiota for which the application of probiotics might be beneficial.