Squamous cell carcinomas of the lung develop through phases of morphologic alterations that involve the pseudostratified ciliated columnar epithelium of the bronchial tree. These changes cover a wide spectrum that includes squamous metaplasia, varying degrees of dysplasia, carcinoma in situ, and early invasion. The development of carcinoma occurs over a prolonged period of time, specifically in chronic tobacco smokers. The cytopathologic changes that represent the preinvasive phases of squamous carcinoma may be detected in sputum several months prior to clinical manifestation of an invasive lesion in screening of high-risk patients. The preinvasive and early invasive changes typically involve large airways and segmental bronchi, and they are not detected radiologically. Patients are often asymptomatic, but they may present with cough and hemoptysis. Early invasive cancers that are not detected radiographically are referred to as occult carcinomas.