Maintenance of steam generators occupies a substantial proportion of scheduled shutdowns at nuclear power stations. Maintenance operations are broken down into a number of distinct phases; these are performed separately to ensure accountability for the work carried out at each stage, thereby guaranteeing the quality of the maintenance process as a whole. One of these phases, known as `marking,' consists in locating certain tubes in the steam generator tube plate and marking them using a suitable system. The list of tubes for marking may be determined on the basis of prior tests. Marked tubes will undergo subsequent operations as required, such as plugging for example. Clearly, the quality of the marking process will have a significant impact on all subsequent maintenance operations on tubes in the secondary bundle. Present-day marking tools make little use of automation, and over-reliance on human judgement means that the marking phase is liable to error. Moreover, depending on the number of tubes to mark, this phase can be long and fastidious. With these considerations in mind, the EDF Research Division has developed a display system for locating steam generator tubes, with the main purpose of facilitating marking operations. Following an initialization phase, this system (named LUCANER) provides the operator with a simple, reliable and fully automatic method for locating tubes in the tube plate. Besides reducing the risk of error, the system also reduces the time required for the marking phase. The system can also be used for complementary phases involving checks on markings, checks on plugging, etc. In a wider context, it provides visual inspection capabilities over a large part of the bowl.