Impression cytology refers to the application of cellulose acetate filter material to the ocular surface to remove the superficial layers of the conjunctival epithelium. Impression cytology has been found to be useful in assessing the ocular surface in various dry eye disorders, such as keratoconjunctivitis sicca (KCS), cicatricial ocular pemphigoid, and vitamin A deficiency. The technique is noninvasive, is easy to perform, causes minimal discomfort to the patient, and can be used to follow changes in the conjunctival ocular surface over time. With this method, the morphology of the conjunctival ocular surface can be studied and the degree of squamous metaplasia assessed. A specific criteria based on the appearance of the epithelial cells and the density of the goblet cells can be used to assign a grade (0-3) to the ocular surface. The grade of the ocular surface is related to the degree of squamous metaplasia and usually parallels the severity of clinical disease. Impression cytology can also be used to differentiate between various dry eye disorders. Disorders that are extrinsic or environmental (such as KCS) often affect the exposed interpalpebral ocular surface before the more protected inferior palpebral ocular surface. Intrinsic surface disorders (such as ocular pemphigoid) affect the palpebral, as well as the bulbar ocular surfaces, early in the disease.