This volume contains a selection of important papers by P.G. de Gennes (1991 Nobel Prize Winner in Physics) which have had a long-lasting impact on the understanding of condensed matter (solid state physics, liquid crystals, polymers, interfaces, wetting and adhesion). A typical example is the original article on "reptation" of polymer chains. The author has added some "afterthoughts" to the main papers (explaining their successes or weaknesses), and some current views on each special problem. Complex systems (polymers or granular matters, and so forth) are explained without heavy calculations, using simple scaling laws as the main tool.