Microscopic colitis is an increasingly common cause of chronic watery diarrhoea, and often a causes of abdominal pain of unknown origins. The increase of interest for this clinical entity is due to a misdiagnosis of any symptoms that have been frequently attributed to diarrhea-predominant irritable bowel syndrome, often for many years before diagnosis. Presumably, most estimates of incidence and prevalence understate the true frequency of microscopic colitis for this reason. The aim of this paper is to evaluate the importance of microscopic colitis as cause of chronic non bloody diarrhoea, on the basis of literature review. These kind of colitis are characterized by normal colonic mucosa at endoscopy or barium enema but with increased inflammation in colonic biopsies. Microscopic colitis consists of two main subtypes, collagenous colitis and lymphocytic colitis, distinguished by the presence of absence of a thickened subepithelial collagen band. Several models of pathogenesis has been proposed but no convincing mechanism has been identified, although is difficult to characterize this clinical entity as an independent phenomenon or a simple manifestation or related factors active to induce microscopic changing in the colonic mucosa. A rational approach to therapy does not exist and was conduct with several types of drugs after the exclusion of other causes, commonly characterized by this symptoms and the definitive histological assessment in the biopsies specimens. In the majority of cases this condition tends to follow a self-limited course but potentially can assume the characteristics of relapsing course with the necessity to a chronic therapy. Several long-term follow-up studies excluded a possible progression to neoplastic malignancies of microscopic colitis.