This book provides in-depth elucidation of several functions of cytokeratins in the organization of the intermediary filaments in normal intestine and liver, as well as microfold L cells and the usability of cytokeratins 7, 8 and 20 in tumor diagnosis. Description of epithelial to mesenchymal transition as a mechanism significant in pathogenesis is also discussed in this book along with the role of cytokeratins for identification of distributed tumor cells and as response markers during chemotherapy. This book is meant for all cancer therapists and researchers who wish to comprehend the diagnostic application of cytokeratins in histology and, particularly, the use of anti-cytokeratin antibodies to recognize viable residual tumor cells accounting for a greater risk of tumor reappearance or cancer cells reacting to chemotherapy, respectively.