Cytokeratins are keratin proteins found in the intracytoplasmic cytoskeleton of epithelial tissue. They are an essential component of intermediate filaments, which help cells resist mechanical stress. The expression of these cytokeratins within epithelial cells is specific primarily to particular organs or tissues. Cytokeratins are particularly useful tools in oncology diagnostics. The expression of cytokeratins varies with epithelial cell type, the extent of differentiation, and the development of the tissue. The cytokeratin patterns are usually maintained during the transformation of normal healthy cells into malignant tumor cells. This enables cytokeratins to be applied as tumor markers. The subsets of cytokeratins that an epithelial cell expresses depend mainly on the type of epithelium, the moment in the course of terminal differentiation, and the stage of development. Thus a specific cytokeratin expression profile allows the identification of epithelial cells. Furthermore, this applies also to the malignant counterparts of the epithelia, carcinomas, as the cytokeratin profile is generally retained. Thus the study of cytokeratin expression by immunohistochemistry techniques is a tool of immense value widely used for tumor diagnosis and characterization in surgical pathology.