In this Nutrition Masters Course on probiotics, a brief history of intestinal microbiome science is presented, including pioneering researchers in the fields of microbiology and immunology. Next, the relationship between the microbiome and human health is explored, with a focus on the gut microbiota and factors that influence the composition or activity of gut bacteria. Specific probiotic strains (e.g. L. salivarius UCC118; B. animalis ssp lactis 420) and their benefits from the literature are detailed, along with emerging areas of microbiome research. Criteria for defining and selecting a quality probiotic are presented per WHO/FAO guidelines.