This entry explores the intricate relationship between the human microbiome, particularly the gut microbiota, and neurodevelopmental disorders such as autism spectrum disorder, attention deficit hyperactivity disorder, and schizophrenia. It highlights the critical role of the gut-brain axis, a communication network involving neural, hormonal, and immune pathways in mediating this connection. This entry discusses how early-life microbial diversity alterations may impact brain function and behavior, potentially through modified immune responses and the generation of microbial metabolites. It also examines emerging microbiome-targeted therapies and the need for future research to uncover causal links and personalized interventions.