This review provides a comprehensive overview of the human gastrointestinal microbiota, detailing the identification and classification of 1057 microbial species across Eukarya (92), Archaea (8), and Bacteria (957). The authors trace the historical development of gastrointestinal microbiota research, highlighting key milestones such as the first cultivation of E. coli and the introduction of anaerobic and molecular techniques. They emphasize the complexity and diversity of the microbiota, noting that many species remain uncultured and uncharacterized. The review also discusses the physiological and genetic characteristics of various bacterial groups, including Actinobacteria, Bacteroidetes, and Firmicutes, and their roles in health and disease. The authors advocate for further research using high-throughput approaches to advance understanding of the gastrointestinal microbiota's functions and interactions with the host.This review provides a comprehensive overview of the human gastrointestinal microbiota, detailing the identification and classification of 1057 microbial species across Eukarya (92), Archaea (8), and Bacteria (957). The authors trace the historical development of gastrointestinal microbiota research, highlighting key milestones such as the first cultivation of E. coli and the introduction of anaerobic and molecular techniques. They emphasize the complexity and diversity of the microbiota, noting that many species remain uncultured and uncharacterized. The review also discusses the physiological and genetic characteristics of various bacterial groups, including Actinobacteria, Bacteroidetes, and Firmicutes, and their roles in health and disease. The authors advocate for further research using high-throughput approaches to advance understanding of the gastrointestinal microbiota's functions and interactions with the host.