Preinvasive lesions are considered the precursors of squamous cell carcinoma of the bronchus. Treatment at the preinvasive stage, before the potential for metastasis, may improve survival from squamous cell carcinoma. An understanding of the natural history and outcome of preinvasive lesions is essential for the accurate interpretation studies of their treatment, and decisions regarding the management of individual lesions. The natural history of preinvasive lesions has only been reported in a small number of highly selected patients and uses different inclusion criteria, treatment criteria. and time-periods of follow-up, making it difficult to draw definitive conclusions. High-grade preinvasive lesions carry a risk of progression to carcinoma but most patients have multiple lesions and a significant probability of developing new lesions over time. Distinguishing lesions with malignant potential, the targets for therapy, from those that will regress or remain indolent is difficult. The American College of Chest Physicians guidelines recommend bronchoscopic follow-up of severe dysplasia and carcinoma-in situ. This review of the evidence regarding the natural history and outcome of preinvasive lesions supports this view, but also shows that further studies in individuals at risk for lung cancer are necessary before guidelines for the management of preinvasive lesions can be developed.