The first three years of life represent a critical window for gut microbiome development, marked by dynamic strain turnover and immune system maturation. During this period, environmental exposures such as dietary transitions, particularly solid food introduction, daycare attendance, infection, seasonality, and household microbial exchange may significantly influence microbial colonization patterns and succession. To investigate how these early life factors influence microbiome assembly, we will longitudinally profile gut microbiota of 500 children at four time points over three years from a Swedish cohort. Using faecal shotgun metagenomics alongside extensive metadata on diet, health, family environment and lifestyle, we aim to investigate how early-life exposures shape microbial assembly trajectories. We will examine alteration in taxonomic and functional capacities and their association with dietary and environmental variables. We hypothesize that earlier and more diverse exposures accelerate microbiota maturation, favouring a rapid transition to adult-like community structures. This study will provide an extensive strain-level, functionally annotated, longitudinal views of gut microbiome maturation trajectories in early life. The findings will advance understanding of how and which early exposures drive microbial succession and may inform strategies for optimizing microbiome development during this critical window of growth and immune training.