Probiotics are now emerging as an important category of food supplement in the United States. Questions about the biologic nature, available products, claimed health benefits, and safety and regulation of probiotics are important for both consumers and nutrition professionals. Probiotics can be considered functional foods because they provide health benefits beyond the traditional nutrition function. With few exceptions, most probiotic products currently available contain lactic-acid-producing bacteria, which mainly belong to the genera Lactobacillus and Bifidobacterium. An adequate level of viable bacteria in a probiotic product and an appropriate daily dose are critical to achieve a health benefit. Because probiotics are not known to be pathogenic and their upper tolerable level is high, they could be promoted as a beneficial food supplement.