The microbiome plays an important role in medicine. In psychiatry, it is also useful to examine the microbiome in relation to the pathogenesis and treatment of psychopathology. Nowadays, during consultation, patients and their relatives more frequently ask questions regarding the microbiome, as well as microbiome-based therapies.<br/> AIM: To give an overview of the current knowledge regarding the relationship between microbiome, behaviour and psychiatric disorders in general, and autism in particular.<br/> METHOD: A narrative literature review based on searches in the PubMed and psycinfo databases with the keywords: microbiota, microbiome, microorganisms, mental disorders, psychiatric disorder, autism spectrum disorder, autistic disorder, autistic and autism.<br/> RESULTS: The number of publications concerning the bidirectional relationship between gut microbiota composition and behavior is considerable. The composition of the gut microbiome affects human behavior via subtle, finely regulated, system biological bidirectional influence. Placebo-controlled research into the effects of microbiome interventions is currently limited.<br/> CONCLUSION: It is too early to make definitive statements about the possibilities of diagnosis and therapy aimed at the microbiome in psychiatric disorders. Further scientific research is necessary. How microbiota play a crucial role in host biochemical homeostasis is, however, becoming increasingly clearer.