Probiotics are living microorganisms that, when consumed, have the potential to confer a beneficial health effect. Unfortunately for purveyors of probiotic products, the system of regulation delineated in the Food, Drug, and Cosmetic Act is anything but "one size fits all." How a probiotic product is used or is intended to be used will govern the regulatory category or categories that the US Food and Drug Administration (FDA) will assign to the product. The extent and nature of the restraints and data-collection requirements that may be imposed on the marketing of a product hinge on how a product is categorized under the Act. More specifically, the categorization of a product governs the respective regulatory burdens of an industry sponsor and the FDA. Premarket systems, such as those for new drugs and biologics, place a heavy evidentiary burden on the sponsor of a product. Postmarket systems, such as those for dietary supplements, place, at least initially, a higher regulatory evidentiary burden on the FDA than on the product sponsor. This article explains regulatory categorizations under the Food, Drug, and Cosmetic Act and their effects regarding the federal regulation of probiotic products.