Lung cancer arises after a series of morphological changes, which take several years to progress from normal epithelium to invasive cancer. The morphological changes progress from hyperplasia, to metaplasia, to dysplasia, to carcinoma in situ, to invasive cancer and finally to metastatic cancer. Multiple molecular changes have been documented in lung cancers, both small cell (SCLC) and non-small cell (NSCLC) types. The number of changes has been estimated to be in double digits. These changes include activation of dominant oncogenes myc family, (K-ras and neu genes), as well as loss of recessive growth regulatory genes or anti-oncogenes (p53, and RB as well as unidentified gene or genes on chromosome 3). However, cytogenetic and molecular genetic studies indicate that multiple other specific sites of actual or potential DNA loss may be present in lung cancers. Other changes may include development of drug resistance, and production of growth factors and their receptors. It is tempting to associate specific molecular changes with specific morphological changes, as has been attempted in the colon. However, because of the difficulties in serially sampling the respiratory tract, such studies have not been performed to date. Documentation of molecular changes in premalignant lesions and prospective studies of their prognostic effects will be necessary for the design of rational chemoprevention trials.