Probiotics have been in widespread use since ancient times and are increasingly being consumed to maintain health and to prevent and treat a wide range of conditions. In the United States, probiotics are considered to be foods or biologics, depending on their intended use. This article addresses the similarities and differences between approaches to conducting clinical trials of probiotics as foods (which leads to health claims) or as biologics (which leads to therapeutic claims). Most probiotics are manufactured as foods, which makes it challenging for academic investigators in the United States to meet the requirements of an Investigational New Drug application that enables them to study the therapeutic effects of these novel agents. Although it is important to ensure the safety and quality of probiotic products, there also may be value in adapting the US Food and Drug Administration's Guidance for Industry for Botanical Products to probiotic products, in part to allow the research agenda to move forward with products for which there are no safety concerns.