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Squamous dysplasia in the bronchi has been long recognized as a precursor of lung carcinoma, particularly squamous carcinoma. Atypical adenomatous hyperplasia (AAH) has been recently implicated as a precursor to adenocarcinoma, Bronchiolar neuroendocrine cell hyperplasia has also been suggested as a precursor to some pulmonary carcinoid tumors. The atypical adenomatous hyperplasia-adenocarcinoma sequence has been likened to the adenoma-carcinoma sequence in the large intestine. AAH is commonly multifocal, and may explain multicentricity that is observed with some adenocarcinomas. AAH has been shown to have immunohistochemical, morphometric, flow cytometric and genetic abnormalities overlapping with adenocarcinoma. Bronchiolar neuroendocrine cell hyperplasia (carcinoid tumorlets) is classically associated with inflammatory lesions in the airways, but may also be multifocal and bilateral. In the latter setting, lesions may attain a size greater than 0.5 cm and be (arbitrarily) classified as carcinoid tumors.