In this session, morphological, biological, and some clinical characteristics of precancerous lesions of the upper aerodigestive tract are described, as are the results of some cancer detection screening studies. Dr. John D. Crissman distinguishes between keratinizing and non-keratinizing dysplasia. Keratinizing dysplasia is identified clinically as the white lesion of leukoplakia, and occurs in the thick mucosa of the hard palate, alveolar ridges, and vocal cords. Non-keratinizing dysplasia is identified clinically as the red lesion of erythroplakia and histologically resembles the classic CIN lesion of the cervix. It is found in the thin mucosa of the floor of the mouth, undersurface of the tongue, inner cheek, pharynx, and upper larynx. Interestingly, in keratinizing dysplasia, individual cells undergo a disregulated switch from small to large molecular weight keratin production and histologically begin to show premature keratinization while still in the lower epithelium. Dr. Crissman presents a useful glossary of definitions used by pathologists when they describe preinvasive neoplasia, and emphasizes that while "dysplasia" refers to morphological changes at the cellular level, "squamous intraepithelial neoplasia (SIN)" refers to the entire biological phenomenon of