This article is devoted to the theme of the etiology of irritable bowel syndrome. This topic is highlighted in comparison with the Roman diagnostic criteria of III and IV revision. The following is considered: the theory of disturbed interaction within the «brain-gut» axis and visceral hypersensitivity induced in this way, as well as a possible mechanism for its implementation; also, the relationship of symptoms of IBS with patients’ mental state: anxiety, depression, sleep disorders. The relationship of IBS with microbiota of the intestine, the genetic mediation of IBS, the examples of candidate genes in biomarkers; the mechanisms of work of these genes are also described; the issue of intolerance to certain foods (FODMAPs, IgE-mediated hypersensitivity) in patients with IBS is highlighted as well; immunological aspects, including the theory of subclinical inflammation (low-grade inflammation) or post-infection IBS.