The diagnosis of celiac disease (CD) is based upon histological findings in duodenal or jejunal biopsies. In the past few years it has turned out that the development of CD lesion in the small bowel is a dynamic process which may present in various histological forms. At one end of the spectrum is a mucosa with normal architecture and an increase in intraepithelial lymphocytes (IEL), at the other end is the classical flat mucosa. Histological features supporting the diagnosis of CD are architectural changes of the villi and/or crypts, an increase in lamina propria cell density and an increase in IEL counts. For diagnostic purposes and for monitoring of CD patients an exact histological classification of the histological findings has to be given. This has become possible by using a modified Marsh classification. In the present paper the histological presentation of CD is presented as well as the modified Marsh classification and the most important differential diagnoses.