The gut microbiota consists of multiple microorganisms including bacteria, viruses and fungi and is referred to as the largest organ of the human body. The relationship between the host and its microbiota is of a mutualistic kind. In physiological conditions, microbiota produce vitamins, enable harvesting energy from undigested carbohydrates and ensure valid functioning of host immunity. The complicated network of connections and signalling, the so called cross-talk, warrants the metabolic and immunological homeostasis of the host. There is emerging evidence that the microbiota is the key component of the gut-brain axis, and its composition provides undisturbed functioning of the central nervous system (CNS) through various mechanisms, including neuronal pathways, production of bacterial metabolites and immune mediators. Alteration in gut microbiota has been linked to an occurrence of various neuropsychiatric disorders, i.e. autism, schizophrenia and depression. The gut-brain axis is the subject of many preclinical and clinical studies, as it is assumed that the insight into the underlying mechanisms of these complex interactions could be beneficial in development of novel therapeutic strategies for the treatment of neuropsychiatric conditions. This review summarizes literature regarding the gut-brain axis, its functioning and its role in the pathogenesis of selected neuropsychiatric illnesses as well as describes possible therapeutic approaches.