Current data on eubiosis and dysbiosis of gastrointestinal tract are discussed along with the role of its microflora in human body under normal and pathological conditions. Certain debatable problems are discussed. Classification of colonic dysbiosis is presented with reference to its stages, functions of normal flora, "myths" related to the science of eubiosis and dysbiosis, the authors views of the problem. Diagnostic methods and their informative value are described. The main diseases and syndromes associated with intestinal dysbiosis are discussed. In conjunction with approaches to its correction.