AbstractEpithelia can be defined morphologically as tissues that line surfaces, and ultrastructurally with reference to their cells' apico‐basal polarity and possession of specific cell‐cell junctions. Defining the epithelial phenotype at a molecular level is more problematic ‐ while it is easy to name proteins (e.g. keratins) expressed by a “typical” epithelium, no known molecules are expressed by every epithelium but by no other tissues. Cells can differentiate to and from the epithelial state as part of normal development, as a response to disease or when manipulated in culture. Many factors (matrix components, adhesion molecules, growth factors, transcription factors) have been identified that can trigger these transitions of phenotype in specific cases, but to date no general master regulators of the epithelial state have been found. The epithelial state may therefore be controlled by multiple regulatory genes so that there is no single molecule responsible for all of the diverse types of epithelium that exist in higher animals.