Increased thickness of the epithelium imparts a white appearance to the oral mucosa by increasing the distance to the underlying blood vessels. Usually this thickening is a result of the increased formation of keratin. Some other less common causes of white lesions are acanthosis or a thickening of the spinous cell layer, edema of the epithelium, or increased fibrosis of the connective tissue thereby reducing blood vessels. Occasionally the surface of an ulcer may appear white, due to collection of fibrin on the surface. In this article the authors discuss white lesions based on putative etiology, that is, hereditary, reactive, inflammation related, immunologic, traumatic, infection related, and idiopathic.