The human body hosts a large amount of microbial species which co-evolved in a mutualistic trend. Most of the microbes live in the intestine where they profit from taking persistent provisions of nutrients and collaborate to produce essential vitamins (such as vitamin B and K), essential amino acids, digestion of carbohydrates and lipids which act as signaling molecules such as short chain fatty acids and secondary bile acids. Beside food digestion gut microbiota is involved in several relevant physiological processes including maturation of the intestinal barrier and regulation of innate immunity. The gut microbiota is constituted by two main bacterial phyla, Bacteroidetes or Firmicutes, altogether >90% of the taxa present in the gut, with a lesser richness of Actinobacteria, Cyanobacteria, Fusobacteria, Proteobacteria, and Verrumicrobia. Despite its functional effects, reports from the last years have demonstrated that the gut microbiota is correlated with several metabolic disorders including obesity, type 2 diabetes mellitus, non alcoholic fatty liver disease and atherosclerosis. The Firmicutes/Bacteroidetes ratio has been shown to associate with obesity and insulin resistance in several studies although with differences according to ethnicity and diet. More recently, measurement of bacterial richness or metagenomic clusters revealed that metabolic disorders are associated to lower richness or depletion of specific clusters. Recent reports outlined a role for reduced occurrence of butyrate producing bacteria such as Clostridiales, Eubacterium rectale, Faecalibacterium prausnitzii and Roseburia intestinalis as well as increased Akkermansia mucinophilia in type 2 diabetes. Other bacterial species such as Prevotella Copri and Bacteroidetes Vulgatus were also identified as relevant for diabetes and insulin resistance although with controversial results. Overall, despite our knowledge of the gut microbiome is still incomplete we can consider it as a new risk factor to be included in the many underlying diabetes