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 our understanding of condensed matter (solid state physics, liquid crystals, polymers, interfaces, wetting and adhesion). A typical example is the original article on "e;reptation"e; of polymer chains. The author has added some "e;afterthoughts"e; to the main papers (explaining their successes or weaknesses), and some current views on each special problem. Complex systems (polymers or granular matters, etc) are explained without heavy calculations - using simple scaling laws as the main tool.