Accurate staging of lung cancers is important to determine the treatment options and the prognosis of patients with a lung cancer. TNM system revised in 1997 by American Joint Committee on Cancer and the Union Internationale Contre le Cancer is widely used in staging of the lung cancer. The TNM system is an expression of the anatomic extent of diseases and is based on the assessment of three components; extent of the primary tumor (T), regional lymph node metastasis (N), and distant metastasis (M). Non-invasive staging of lung cancers is based primarily on chest computed tomography (CT), and if available, on positron emission tomography (PET). Chest CT scanning is useful in providing anatomic details, but the accuracy of the chest CT scanning in differentiating benign from malignant lymph nodes in the mediastinum is poor. PET scanning has a much better sensitivity and specificity than chest CT scanning for mediastinal lymph node staging, and distant metastatic diseases can be detected by PET scanning. With either test, abnormal findings must be confirmed by a tissue biopsy to ensure accurate staging. Invasive techniques for biopsy of mediastinal lymph nodes or pathologic tissue include transbronchial needle aspiration, transesophageal fine needle aspiration, and surgery.