This lecture delves into the intricate world of the human gut microbiota, exploring its composition, functions, and impact on health. From the vast array of microbial species to the techniques used for analysis, the lecture covers the mutualistic relationship between the microbiota and the human host. It discusses the variations in microbiota among individuals, the role of microbiota in diseases like obesity and inflammatory bowel disease, and the potential of microbiota therapy. The lecture also touches upon the evolutionary aspects of microbial symbiosis and the implications of dysbiosis in chronic immune diseases.