Neuropsychiatric disorders are one of the most common problems in the modern world. In the European Union alone, at least 164 million cases of mental problems were reported in 2016. Despite many years of investigations and enormous resources devoted to scientific research, the causes of some of these diseases have not been clearly identified, and, in the case of others, treatment options are significantly limited. Although the brain weight accounts only for about 2 percent of the body weight, its oxygen consumption accounts for up to 20% of the resting energy metabolism. As opposed to other organs, the brain does not accumulate energy; therefore its functioning depends on the continuous supply of the main energetic substrate, i.e. glucose. At the same time, the physiology of every cell in the human body depends on the availability of regulatory components, including vitamins and trace elements, which are derived from the diet. As evidenced by the basics of cell biology, certain nutrients are essential for proper neuronal functioning and, consequently, nerve signal transmission. Nutrients are also known to be metabolically utilised by intestinal microorganisms, and the neuroactive and immunocompetent products of these transformations pass through the gut–brain axis into the brain and exert their effects on the structure and function of regions responsible for emotions as well as motor and cognitive abilities. The first part of the article presents information regarding intestinal microbiota as a structural and functional nervous system modulator. Particular attention is paid to intestinal dysbiosis in neurodevelopmental and neurodegenerative disorders, which is currently considered to be the cause and/or a factor determining their clinical course. It was also suggested that supporting gut microbiota with probiotic therapy may be an element of nervous system disease prevention and support the primary treatment of these conditions.