When considering three-dimensional conformal radiotherapy for non-small cell lung cancer, the uncertainties about treatment can be quite significant, due to set-up errors and organ or tumor motion. These can be important causes of treatment failure. Immobilization devices have only been studied recently in a scientific manner in the domain of chest tumors, presumably because other factors such as tumor motion were felt to be more important causes of treatment uncertainties. An international survey on immobilization devices in the treatment of non-small cell lung cancer has shown that about half of the centers are using three-dimensional conformal radiotherapy, and among these, only two-thirds use immobilization devices on a routine basis. Very few use internal fiducials. Current data on set-up errors show that the average discrepancy is about 5 mm, but in some cases it can be more than 15 mm. A recent study has demonstrated that less positioning corrections during treatments were needed for the patients who were immobilized. Another work indicates that there were no differences between a T-bar immobilization device and a system using chemical foams. Other works indicate that internal motion of bronchial tumors can vary greatly, depending on their location. A number of clinical groups are looking at minimizing the consequences of internal motion, but the currently proposed techniques appear to be cumbersome. New studies will be necessary in order to improve the knowledge of daily positioning and the effect of internal motion. Until then, it is essential to take care when considering narrow margins in conformal radiotherapy of non-small cell lung cancer.