Microscopic examination of stained smears and tissue sections remains the standard method for definitive diagnosis and classification of lung cancer. However, the morphology of lung cancer is complex, and consensus classifications such as those prepared by a panel World Health Organization (WHO) are required for the sake of consistency and clinical relevance. In the most recent (1999) WHO classification, the diagnostic categories of greatest clinical importance, small cell lung carcinoma and non-small cell lung carcinoma, remain fundamentally unchanged. However, application of immunohistochemistry and electron microscopy has revealed expression of neuroendocrine markers in a wide variety of tumors. Expression of these markers is not taken into account in current treatment protocols, and additional correlative studies will be required to determine the clinical relevance of neuroendocrine differentiation in lung carcinoma. In addition to histological classification, microscopic analysis can providein situevidence of response to chemotherapy, as well as information on precursor lesions and multistep carcinogenesis in the airways. Finally, it is likely that morphological assessment of lung carcinoma and preneoplastic lesions will continue to be refined as new diagnostic modalities such as spiral computed tomography and fluorescence bronchoscopy provide previously inaccessible specimens for morphological and correlative molecular studies.