The human intestine is colonized by a large number of microorganisms that inhabit the intestinal tract and support a variety of physiological functions. The stepwise microbial colonization of the intestine begins at birth and continues during the early phases of life to form an intestinal microbiota that is different for each individual subject. This process facilitates the formation of a physical and immunologic barrier between the host and the environment, helping the gastrointestinal tract maintain a disease-free state. Probiotics are viable microbial food supplements that have a beneficial impact on human health. Health-promoting properties have been demonstrated for specific probiotic products. Scientific data are accumulating on these properties, especially in infants; the most significant effects include prevention and treatment of antibiotic-associated diarrhea and rotavirus diarrhea and allergy prevention. Bifidobacteria appear to be the most promising probiotic candidates, followed by defined lactic acid bacteria, which favor specific healthy bifidobacterial growth and species composition. Because viability appears to be important, probiotic properties also should be emphasized to meet this criterion. For future probiotics, the most important requirements include a demonstrated clinical benefit supported by mechanistic understanding of the effect on target population microbiota and immune functions. Genomic information and improved knowledge of microbiotic composition and its aberrancies should serve as a basis for selecting new probiotics for use in specific infant populations.