The city planning of La Chaux-de-Fonds, they way we know it today, is the result of a long evolution. It starts the day after a fire which, in 1794, practically devastated the whole village. With the human and material loss which the fire caused, this disaster offered the possibility to reconstruct on a new basis. The fear of fire and the demands of the watch industry, will dictate the policy of city planners and architects. As of this event, the urban expansion of La Chaux-de-Fonds is characterised by three major stages. All three are continuous and interrelated. Each is the product of a particular person. Their coming corresponds to a precise period in time and a given context. The resulting "urban matrix", called massif, will serve for a long period of time as a model for the extension of the city. The continuous use of this model will serve until to the beginning of the 20th century and will only be abandoned at the end of the 1st world war. The present study comprises two distinctive parts, complementary and of equal importance: - A written part and - An illustrated part The choice of these two means for the research is to insure a better understanding of complex subject: - Stage 1: 1794-1795: The reconstruction of the burned down village is assigned to Moïse Perret-Gentil (1744-1815) with the introduction of the first elements of orthogonal planning. It initiates the system of the grid plan which will define future works. - Stage 2: 1831-1835: The continuous and accelerated development of the watch industry pushes the local authorities to hire the engineer Charles-Henri Junod (1795-1843) to plan an extension at a larger scale. The document is named the "General Plan of Alignments". Thanks to its singularity and its urban impact, the Junod plan occupies a significant place in this doctorate thesis. - Stage 3: 1854-1859: The arrival of the railroad, the Industrial Jura, will profoundly modify the urban configuration. The intervention of Charles-Frédéric Knab (1822- 1874) is based on the works of his predecessor. It includes the railroad as a complete part of his Plan of Alignments. It results from this study that the city plans of La Chaux-de-Fonds conceived by the engineers Charles Junod and Charles Knab are not just a simple grid plan applied at random. Their characteristics are far from the image of a checker board plan to which La Chaux-de-Fonds is usually associated to. Instead, these plans result respectively from a combination of refined imbrications of grids which perfectly connect to the existing urban tissue. Behind their apparent regularity, the Junod and Knab plans, hold a series of irregularities which insure at the same time their complexity and their richness. The works of these two engineers constitutes an exemplary case of an urban transformation, a model for a remarkable case of city planning.