A histological classification should provide guidelines for tumor diagnosis in order to evaluate patient prognosis and therapy. Pre-invasive lesions identified as precursors of invasive lung carcinoma are: squamous dysplasia/carcinoma in situ, atypical adenomatous hyperplasia and idiopatic pulmonary neuroendocrine cell hyperplasia. Squamous carcinoma and adenocarcinoma are the commonest types of lung carcinoma with the latter increasing in many countries mainly for changed smoking habits. Bronchioloalveolar carcinomas include exclusively noninvasive mucinous or non-mucinous tumors. Neuroendocrine tumors range from well differentiated neuroendocrine carcinoma (typical carcinoid) to intermediate grade (atypical carcinoma) to very aggressive poorly differentiated lesions (large cell neuroendocrine carcinoma and small cell carcinoma).