Complete and accurate classification of all cases of cancer is essential for planning optimal management, estimating prognosis, communicating about individual patients or groups of patients, comparing results of various treatments and evaluating cancer control programs. For most types of cancer, the stage, or the anatomic extent of the cancer, is as important as other characteristics such as the primary site of the tumor, the histologic cell type, and the grade of the malignancy. The TNM system of staging recommended by the American Joint Committee on Cancer and the International Union against Cancer is used throughout the world, and there is convincing evidence of its value in selecting treatment and estimating prognosis. For example, Stage I squamous cell or adenocarcinoma of the lung is almost always amenable to surgical resection and five-year survival rates of about 70% are being reported by many cancer centers. In contrast, stage III lung cancer is usually not resectable and the five-year survival rate of this group is less than 10%.