Prognosis for non-small cell lung cancer (NSCLC) patients who have locally advanced unresectable disease is poor for persistent thoracic disease and development of distant metastasis. In view of the poor rate of local control following conventional radiation therapy, there is a great need for methods to improve its efficacy. Three-dimensional conformal radiation therapy (3DCRT) is a high precision radiotherapy able to deliver higher doses with smaller doses to the surrounding normal tissues. However, the real problem is: "what do I want to treat?" This question is addressed based on the clinical and biological target volume definition. Selection of the CTV is therefore basically the clinical compromise between the most radical possible CTV and the critical normal tissue tolerance. The clinical target volume includes the GTV plus a margin to encompass subclinical or microscopic malignant disease immediately adjacent to it. Standard radiation therapy consists of a dose of 40 Gy to the entire mediastinum, supraclavicular fossa, and ipsilateral hilum, even if there is no evidence of disease in these areas. Despite the high risk of nodal spread in lung cancer, the benefit of additional elective nodal irradiation (ENI) is not proven while it seems to significantly increase the rate of radiation morbidity. Several studies have been published where ENI was systematically omitted. The main arguments for omitting ENI and the principal clinical experiences, are discussed.