The human gut microbiota is a complex community of microbes that influences health and disease through its metabolic activities and interactions with the host. Understanding the factors that shape the gut microbiota is crucial for developing therapies targeting it. The gut microbiota is highly diverse, varies between individuals, and can fluctuate over time, especially during disease and early development. Ecological principles can help understand host-microbe interactions and the functions of the gut microbiota. Advances in sequencing technologies and omics data are improving our understanding of how host and environmental factors affect the gut microbiota over a lifetime. The composition, diversity, and function of gut microbial communities have the potential to inform personalized nutritional and drug treatment strategies. The gut microbiota is highly variable, with a core microbiota shared by most healthy adults. However, the diversity of the microbiota is highly variable over time and across populations. Each individual harbors more than 1,000 'species-level' phylotypes. The main phyla in adults are Bacteroidetes and Firmicutes, while other phyla are generally minor constituents. The gut microbiota contains pathways for central, carbohydrate, and amino-acid metabolism, but not all pathways are represented. Functional diversity in the gut microbiota is influenced by factors such as diet, environment, and genetics. The functional gene profiles are quite similar across individuals, suggesting a functional core microbiome. Age, genetics, environment, and diet are factors that drive normal variation in the gut microbiota. The microbiota of infants is more volatile and varies more than in adults. The microbiota of infants is affected by antibiotic use, breastfeeding, and mode of delivery. Differences in microbiota composition in early life may affect susceptibility to immunological diseases into adulthood. The microbiota of infants is affected by antibiotic use, breastfeeding, and mode of delivery, but the extent to which these differences affect the adult microbiota is not well understood. The gut microbiota is influenced by factors such as the environment and diet, which are often confounded. Twins and mother-daughter pairs have more similar microbiota compositions than unrelated individuals, suggesting a genetic influence. However, monozygotic and dizygotic adult twins have equally similar microbiota, suggesting that environment rather than genetics may drive familial similarities. Populations can be separated by characteristic differences in the gut microbiota. For example, Italian children have a different microbiota from children from rural Africa, and both children and adults from the United States have a very different microbiota from populations in Malawi and the Amazonas state of Venezuela. The microbiota variability is linked to human health and medicine. Differences in the microbiota and microbiome could help explain the variation in gut metabolic processes of individuals, including the metabolism of drugs and food. The health benefits of diets rich in soya, such as improvements in vasomotor symptoms, osteoporosis, prostate cancer, and cardiovascular disease, have been attributed to (S)-equol producedThe human gut microbiota is a complex community of microbes that influences health and disease through its metabolic activities and interactions with the host. Understanding the factors that shape the gut microbiota is crucial for developing therapies targeting it. The gut microbiota is highly diverse, varies between individuals, and can fluctuate over time, especially during disease and early development. Ecological principles can help understand host-microbe interactions and the functions of the gut microbiota. Advances in sequencing technologies and omics data are improving our understanding of how host and environmental factors affect the gut microbiota over a lifetime. The composition, diversity, and function of gut microbial communities have the potential to inform personalized nutritional and drug treatment strategies. The gut microbiota is highly variable, with a core microbiota shared by most healthy adults. However, the diversity of the microbiota is highly variable over time and across populations. Each individual harbors more than 1,000 'species-level' phylotypes. The main phyla in adults are Bacteroidetes and Firmicutes, while other phyla are generally minor constituents. The gut microbiota contains pathways for central, carbohydrate, and amino-acid metabolism, but not all pathways are represented. Functional diversity in the gut microbiota is influenced by factors such as diet, environment, and genetics. The functional gene profiles are quite similar across individuals, suggesting a functional core microbiome. Age, genetics, environment, and diet are factors that drive normal variation in the gut microbiota. The microbiota of infants is more volatile and varies more than in adults. The microbiota of infants is affected by antibiotic use, breastfeeding, and mode of delivery. Differences in microbiota composition in early life may affect susceptibility to immunological diseases into adulthood. The microbiota of infants is affected by antibiotic use, breastfeeding, and mode of delivery, but the extent to which these differences affect the adult microbiota is not well understood. The gut microbiota is influenced by factors such as the environment and diet, which are often confounded. Twins and mother-daughter pairs have more similar microbiota compositions than unrelated individuals, suggesting a genetic influence. However, monozygotic and dizygotic adult twins have equally similar microbiota, suggesting that environment rather than genetics may drive familial similarities. Populations can be separated by characteristic differences in the gut microbiota. For example, Italian children have a different microbiota from children from rural Africa, and both children and adults from the United States have a very different microbiota from populations in Malawi and the Amazonas state of Venezuela. The microbiota variability is linked to human health and medicine. Differences in the microbiota and microbiome could help explain the variation in gut metabolic processes of individuals, including the metabolism of drugs and food. The health benefits of diets rich in soya, such as improvements in vasomotor symptoms, osteoporosis, prostate cancer, and cardiovascular disease, have been attributed to (S)-equol produced