The gut-brain axis describes the bi-directional signalling between gut microbiota and the central nervous system via the vagus nerve, immune system, and microbial metabolites including short-chain fatty acids and neurotransmitter precursors. A growing body of correlational and small-scale interventional studies links specific microbial compositions to anxiety, depression, and autism-spectrum traits in both humans and animal models. The theory is well-supported as a framework but causal mechanisms and clinically reliable probiotic interventions remain under active investigation. Stronger causal evidence will require larger randomized trials and standardized microbiome characterization.