Squamous cell carcinoma of the lung has been well studied and can be characterized as having a precancerous, preclinical, and clinical phase. In the precancerous phase, it is now known that the development of carcinoma may parallel that of squamous metaplasia, but is fundamentally different and only related by virtue of similar cytoplasmic differentiation. Precancer progresses through dysplastic phases to carcinoma in situ and these changes can be documented by aneuploidy in the abnormal cells. Some of all these degrees of dysplasia, including carcinoma in situ, have been shown to be reversible. In the preclinical (occult) phase, 146 cases have been reported. They range in stage from superficial carcinoma in situ to invasive and metastatic unresectable carcinoma because of their close association with the hilar structures. Even though the best opportunity to cure squamous cell carcinoma is during the preclinical phase and despite the advanced stage it may reach during this phase, squamous cell carcinoma is the most curable of the bronchogenic carcinomas during its clinical phase. It appears that local spread of this tumor (T2) is of greater significance than hilar node metastases (N1) in the absence of local spread by the primary (T1).