Improving the quality of life in developed countries has contributed to an increase in its duration, which has led to an increase in the number of reported cases of Alzheimer’s disease (AD) and Parkinson’s disease (PD) in the world. Today, there are 26.6 million patients with AD in the world and it is suspected that by 2050 the number of such patients may increase four times. Additionally, PD in different countries is recorded among people above 60-65 years old at a level of 167 to 5703 per 100.000 population. The latest studies have made it possible to formulate the main mechanisms of the «microbiota-gut-brain» axis associated with the pathogenesis of some neurodegenerative diseases. In this review, we summarize the currently available information on the possible role of the gut microbiota in the AD and PD development. It was shown that oxidative stress is one of the main pathogenetic mechanisms of the development of neurodegenerative diseases. In addition, the deposition of lipopolysaccharides of gram-negative bacteria and amyloid of microbial origin in the brain tissue of patients with impaired permeability of the intestinal barrier plays an important role in AD. In PD, the synthesis of α-synuclein produced by bacteria and neuroinflammation are of the greatest importance. Knowledge of these mechanisms will allow the development of psychobiotics, which will reduce the risk of neurodegeneration in AD and PD.