The gut–brain axis represents a two-way interaction system connecting the enteric and central nervous systems. This connection is not merely structural; it encompasses various pathways, including neuronal, endocrine, and immune paths. Through the autonomic nervous system, the hypothalamic–pituitary–adrenal (HPA) axis, and gastrointestinal nerves, the gut communicates with the brain, enabling the brain to impact intestinal functions, including immune cell activity, and allowing the gut to influence mood, cognition, and overall mental well-being. This section provides an overview of recent research concerning the microbiota–gut–brain axis and its involvement in the pathophysiology of various mental diseases, encompassing depression, schizophrenia, bipolar disorder, anxiety disorders, autism spectrum disorders, and neurodegenerative diseases.