Nowadays, plasma-etching processes are asked to produce patterns from the nanometer to the micrometer range with the same efficiency. The very severe requirements in terms of etch rate, selectivity, profile control and surface damage plasma-etching processes lead to, have been at the origin of the development of mechanistic studies by means of plasma diagnostics and surface analysis, as well as the development of new etching devices. We review here the basic concepts of plasma etching, and using examples, we describe more in details important features. We recall, in particular, the important role of the surface layer, the ion bombardment and the substrate temperature.