The purpose of the thesis is to fill a gap in the publications about Viollet-le-Duc, outlining his profile as an architect through a chronological study of his projects, whether built or not, of his theoretic writings and, when useful, of his restoration works, to achieve a full vision of his extraordinary contribution. His theoretic and practical work has been studied through a precise bibliographical and documentary research as well as the direct analysis of his architectures and the monuments he restored in various regions of France. Furthermore, his works and writings have been put back in their historical context. The thesis recounts the origin and the development of Viollet-le-Duc's rational vision of Gothic and the growing focus on construction in his conception of architecture, from his education years in France and in Italy, to his maturity. Viollet-le-Duc defines principles deriving from Gothic that act as formal and spatial qualification of the project. The fundamentals of his vision are truth of construction, of structure and of materials, focus on structure in the project process and in the definition of the sculptural and pictorial ornament, polychromy deriving from construction, expressing outside the structure and the interior functions, articulation of volumes. From these principles, Viollet-le-Duc tries to draw a method different from the Beaux-Arts, a truly modern and rational new style. While searching for an alternative to the Beaux-Arts system, Viollet-le-Duc studies the nature and scientific theories of his century. The very analogy with crystals makes it possible for Viollet-le-Duc to conceive a totally new structure in the theoretic projects in masonry and metal. From the morphology of plants and animals, Viollet-le-Duc draws references for the formal definition of metallic structures. Regarding ornament, Viollet-le-Duc pushes to extreme limits the inspiration of nature stemming from the study of medieval examples. His stylized forms with increasingly free lines open the way to Art Nouveau.