Electromagnetic non-destructive evaluation of complex objects means that one has to decipher data, which result from their interaction with imposed sources. This task is crucial in civil, environmental and medical engineering, to quote evident fields, as well as for questions of safety and reliability of industrial processes of various kinds in key energy and transportation sectors, for example. This short contribution does not attempt to review the huge variety of themes and the many applications of the science of inversion, but aims at emphasizing a number of points that seem common enough to this science for being worthwhile to be reminded. Two illustrations and a few main references thought of good interest among the ever increasing literature are given.