This study plans to collect 106,667 fecal samples from Chinese infants and children under 6 years old, with multi-dimensional stratification based on geographic location, mode of delivery, antibiotic exposure, and developmental stage. Fecal gut microbiota will be profiled to construct a gut microbiome atlas, establish quantitative analytical models linking geographic regions, physiological phenotypes and microbial signatures, and identify key microbial biomarkers associated with core healthy phenotypes.