Technical Abstract: Tea leaves produce secondary metabolites, organic compounds that are involved in the defense of the plants against invading pathogens including insects, bacteria, fungi, and viruses. These metabolites include polyphenolic compounds, the six so-called catechins, and the methyl-xanthine alkaloids caffeine, theobromine, and theophylline. Postharvest inactivation of phenol oxidases in green tea leaves prevents oxidation of the catechins that impart the green color to green teas, whereas postharvest enzyme-catalyzed oxidation (fermentation) of catechins in tea leaves results in the formation of four theaflavins as well as polymeric thearubigins which impart the black color to black teas. Partly fermented oolong teas contain both classes of phenolic compounds. Teas from different sources differ widely in their content of flavonoids (polyphenolic catechins plus theaflavins). Interest in tea flavonoids arises from the fact that they are reported to exhibit numerous health-related beneficial effects, including strong antibiotic activities. A need therefore exists to develop a better understanding of the roles of polyphenolic tea compounds in food and medical microbiology. To contribute to this effort, this overview surveys and interprets published studies of activities of tea flavonoids and teas against pathogenic bacteria, virulent protein toxins produced by some of the bacteria, virulent bacteriophages, pathogenic viruses, and fungi. Also covered are synergistic and mechanistic aspects of the antimicrobial effects. Further research is suggested for each of these categories. The described findings are not only of fundamental interest, but also have practical implications for nutrition, food safety, and animal and human health.