Alexandre Sarrasin (1895-1976) studied at Zürich Federal Swiss Institute of Technology (ETH) from 1913 to 1918. Despite First World War's conflicts surrounding the neutral Switzerland, the intellectual climate inside ETH was cosmopolitan and liberal. As unique educational institution directly depending from the Federal government, the ETH played an important role in shaping modern Switzerland where country planning and infrastructure building were the central power's main tasks. Engineers were important figures of this process of modernization. They embodied the values of the industrial revolution: rationality, efficiency and the huge power of transformation of the country and its society. Sarrasin opened an engineer's office in Lausanne in 1921 and in Brussell two years later. He established himself in the capital of Belgium in 1927 while ruling at the same time his office in Lausanne. He came back to Switzerland when the German army invaded Belgium. The office in Bruxells was maintained up to the early 1960's. Branches were opened in Sion and Geneva during the 1950's. Sarrasin's work concerned foundations, buildings, bridges and some dams also. Nearly all these constructions works were made of reinforced concrete, his favorite material. This thesis is dedicated to the construction works, especially the bridges in Valais, Sarrasin's native district, and the aesthetic question in engineer's design. Of the bridge of Branson (1924- 1925) near Fully, his first important commission, he wrote that "The elegance of the design is due to its simplicity. The quietness of its lines is fully harmonized with the landscape. Nothing was planned for the decoration. The bridge is the simple realization of a mathematical truth". This quotation highlights the issue of form for engineers and especially in Sarrasin's works: the affirmation of simplicity and "mathematical truth", that is to say the expression of the structural mechanics, as the engineer's aesthetic's fundamental the relation with the landscape The question of form was not only thought of in terms of efficiency and economy, or lack of decoration. The development of structural mechanics as engineer's science gave new conceptual tools allowing new designs. The first professor of civil engineering at the ETH, Karl Culmann (1821-1881), was the graphical static's father. This applied science was inspired by descriptive geometry and graphical calculation methods developed in France during the 18th and 19th century. To the construction's classical point of view distinguishing between elements bearing weight (for example pillars) and others being supported (for example lintels), graphical static opposed a spatial vision of forces interactions into the structure. New materials allowed bringing these forces into play with tensed cables, compressed or extended frames. At the beginning of the 20th century, reinforced concrete appeared as modern material along side with iron and steel. Created by builders in the 19th century, its construction method was investigated by engineers making it a universal material used by everyone, independent of patents. This assimilation was made by elaborating construction norms that had sometimes inhibitive effects on the creativity of engineers. Other engineer looked for new forms fitting the material's caracteristics. Robert Maillart (1872-1940) was one of them. He had a great influence on Sarrasin's work. The first half-century of modern engineers which started with the creation of the ETH in 1855, was a period of great innovations in science and technology. Most construction works were made of metal and were built for the railways and designed with the graphical static, creating a new architecture, but the question of aesthetic became an issue only at the beginning of the 20th century. The first reaction rose against the overwhelming presence of technology in surroundings : its impact on the landscape or on the city's image was depicted as negative. At the same time, other observers – architects, art historians – recognized in industrial constructions an aesthetical value. Some engineers wanted a come back to traditional forms as stone vaults. During the first quarter of century, several metallic bridges had been considered as fragile and replaced by stone works. Bridges were designed as monuments. On the other side, some engineers argued for the beauty of modern materials and technology, opposed to the picturesque point of view, represented by those who wanted preserving the landscape. The First World War was not only the beginning of the 20th century. It seemed also to be the moment where engineers lost their innocence. Aesthetical questions and the impact on the landscape were matters of public debates. During the same period engineering became a profession free of contractors. That had probably the effect that engineers developed their own language. Sarrasin exactly spent his study years during this period of changes. Matters of aesthetic belonged to the themes treated by his professor of structural mechanics, Arthur Rohn (1878- 1956), who considered that a bridge should not only be built rationally, but should also express his structural system. François Schüle (1860-1925), who was involved in developing constructive norms for reinforced concrete, was his professor of material's technology. He represented the scientific side of the profession of engineer. The works of Sarrasin embodied this double requirement for rationality in using materials and the will to integrate the construction works into the landscape and into the environmental context. Valais was the landscape taken into account. This mountainous district entailed contradictory representations of the need of modernization and the preservation of landscape and traditions. On one side, this region was shown as a society preserved from modernity with unchanged traditions, on the other side, the Valais quickly took part in the national economy. The landscape changed deeply with the draining of the main valley of the Rhône river, the construction of rail and road nets, industry and hydroelectric plants. Sarrasin's works were the products of this economic revolution which transformed the Valais region; they wanted at the same time to be a testimony of this story. One of the interesting aspects of the attitude of this engineer is the way he used references to ancient constructions. Beamers and arches reconsidered in respect with modern technology by use of reinforced concrete, created a dialogue with the landscape or with stoned vaults thrown upon the Alps deep abysses.