Most of the oral soft tissue diseases, however,derived from the oral squamous cells. More than 80% of oral malignancies are squamous cell carcinoma. Oral squamous cell carcinoma （OSCC） sometimes shows the features of oral epithelial dysplasia （OED）, carcinoma in situ （CIS） or oral intraepithelial neoplasia （OIN）. Though cytology has been applied to the diagnosis of oral lesions, the number of false-positive and false-negative diagnosis affected cases may indicate the difficulty in the cytological diagnosis of borderline lesions. Recently, such a borderline lesions has been categorized to potentially malignant disorders （PMDs）, and most of the squamous cell carcinoma are thought to develop from PMDs. This review paper describes this new conception of PMDs from the viewpoints of cytology, distinguishability of PMDs and our surgical approach of oral tumors. Furthermore, relationship between oral and upper digestive tract diseases are discussed.