Inflammatory bowel disease (IBD) is a group of chronic idiopathic inflammatory conditions. The two major types are ulcerative colitis (UC) which affects the colon and Crohn’s disease (CD) which can involve the whole gastrointestinal tract but is most common in the terminal ileum and colon. There is a genetic predisposition for IBD, and patients with this condition are more prone to the development of malignancy. Further types to be considered are indeterminate colitis (IC) and inflammatory bowel disease unclassified (IBDU). These are essentially “temporary diagnoses” when the difference between UC and CD cannot be established definitely at the time of presentation. IC should be used when examination of surgical samples is available together with clinical, serological, and imaging data, while IBDU is used for patients from whom only endoscopic biopsies are available. CD and UC must be considered in the differential diagnosis of clinically acute colitis because of differences in treatment strategies between infections and IBD. The differential diagnosis is particularly important when the complaints are persisting. Histology plays a key role in the diagnosis. Major diagnostic features of UC are architectural distortion and basal plasmacytosis. It has been suggested that “microscopic colitis” would also be member of the chronic inflammatory bowel diseases.