Clinical staging is fundamental in lung cancer, as this descriptive system facilitates the transmission of the extension of the disease, provides prognostic information, helps in choosing the most appropriate treatment program, and provides a standardized system that enables the treatment outcomes of different series of patients to be compared. The most important technique for defining local, regional, and remote extension of the disease continues to be chest and abdominal CT. Functional imaging techniques play a complementary role that is becoming increasingly important in the noninvasive characterization of mediastinal lymph-node involvement.