This thesis is divided into three separate volumes, which are consistent with the indifferent order in which they could be read. Two of them are dedicated to two masters of modern architecture, Mies and Utzon, who respectively personify two major architectural currents which share with unequal intensity the first half of the twentieth century: rationalism and realism or organic architecture. The third volume presents some important platforms of the architecture of Mesopotamia, Egypt and Greece, that shortlist of three architectural cultures which, according to Giedion, embodied the first spatial conception of architecture. This volume is presented here as an annex, as this thesis mainly analyzes various notions of platforms built during the twentieth century. But it was precisely from the reinterpretation of the platform, classical in essence, that those magnificent massive platforms of the antiquity rightly claimed their modern relevance. Both Mies and Utzon not only delimited their respective projects between two substantially horizontal layers, that of the ground and that of the roof, but also the formal characteristics of each of these episodes defined the key attributes and interests of their architecture. In both cases, however, it has been the plane of the roof which has raised more analysis and recognition: - In the case of Mies, his roofs, straight and flat without exception, have been the result of the evolution of certain structural typologies, in concrete and later on in steel, which also fulfilled a conception of programme that prioritized flexibility ahead of functionality . - In the case of Utzon, his deliberately wavy roofs, invariably of concrete, have been the most distinctive figurative feature of all his architecture, eclipsed under the impact of his most celebrated work, the Sydney Opera House. However, for the understanding of the architecture, of both Mies and Utzon, it would be erroneous not to put the episodes of their respective roofs in relation to their ground planes, mostly understood as platforms. It is necessary to believe therefore that the numerous essays that have focused on the top plane, that of the roof as the preferred objective of analysis or even as the only one, of their respective works, have done so knowing this dual relationship, assuming this reductionism as a strategy for a greater analytical depth. This work poses a similar license, focused in this case, however, on the other horizontal plane, the one underneath, the ground level, significantly less explored and, in the opinion of the author, equally relevant. From this reduction of the focal field, where the platform becomes the epicentre of the projects of both architects, this thesis has been carried out to state that: -Both in the case of Mies and in the one of Utzon, the use of the platform, far from being an isolated fact, became a constant in each of their respective careers. - Except for a few exceptions, determined by singular requirements of the environment or the programme, all of their respective platforms are massive, far from the light horizontal planes with which modern architecture was built. - Both in one case as in the other, the platform is systematically used as a mediation space, to interact with the environment and determine the limits of the project and as transition space (where one passes), to build the visual approach system of the compound. -Starting from a certain moment, corresponding to the mature stage of both architects, Platform and Access united an inseparable binomial, prevalent in the construction of ground floors of their respective architectures, which gave place to an episode, that of the elevated access, which characterized all of their work. The platform became the threshold of their architectures.