The International Staging System for Lung Cancer provides for classification of six levels of disease extent in five stage groups that relate to patient management and prognosis. Stage 0 is reserved for patients with carcinoma in situ. The Stage I and II definitions provide for classification of two levels of disease extent completely contained within the lung that have different prognostic and therapeutic implications. Definitive resection is the first choice of therapy for patients with non-small cell lung cancer in these stage groups. The Stage II category takes into account the erosion of survival expectations in the optimum group of T1 and T2 patients as a consequence of intrapulmonary lymph node involvement. Although small cell carcinoma is infrequently encountered as Stage I and Stage II disease, these classifications may be useful in the structure of investigational programs involving adjuvant surgery. The exclusion of distant metastases and the division of Stage III into two levels of extrapulmonary disease allow for selection of patients for specific treatment plans. Patients with non-small cell tumors with Stage IIIa disease usually are candidates for definitive surgical treatment. The specificity of the T and N definitions in the Stage IIIa and IIIb categories identifies patients for whom particular radiotherapy treatment plans are structured and protocol assignments are made. It is consistent with patient management concepts that all those with distant metastases are classified as having Stage IV disease. Implications of the system for selection of surgical, radiotherapeutic, and chemotherapeutic regimens are rational for all cell types. The classification meets the requirement for simplicity and can be readily applied in a broad spectrum of clinical and teaching environments. It is, however, sufficiently specific to be useful for reporting results of investigational therapies. Prospective use of the classification should encourage precision in clinical evaluations that exploit full use of refinements in imaging technologies. The cooperative efforts of the Task Force on Lung of the AJCC and the TNM Committees of the UICC to bring this classification system to fruition and international acceptance have been described. It has been adopted by these groups and others, including the International Association for the Study of Lung Cancer, the Japanese Cancer Committee, and the Spanish Society of Respiratory Disease, as their official recommendation for staging lung cancer.