Architectural principle according to which the form of a building is to be derived from the function it is intended to fulfil; the schematic and technological aspect of architectural modernism (Rationalism), whose wider theoretical stance comprises also philosophical, political, social, economic, stylistic and symbolical questions. Functionalism in architecture remains in part the essence of the modem as opposed to the traditional. Therefore, there is hardly an architectural principle which occurs with greater persistence in the history of architecture, nor one which is less appropriate to characterize any chronologically delimited movement. Even in palaeolithic cave dwellings and in neolithic lake dwellings form was determined by function; in Roman fortifications and aqueducts, in medieval castles, in Renaissance palaces and Baroque country houses, in18th-century warehouses, in residential architecture of the 19th century and office skyscrapers of the 20th. there is a close relationship between form and function. Functionalism is as old as building itself.

 Parallel to that, the theoretical basic of Functionalism also goes back to the beginnings of architectural theory: thus, Vitruvius insisted that the form of a structure must be derived from its intended use. Functionalist postulates reappear from then on, above all in the rationalist treatises of the 18th century by Carlo Lodoli, Marc-Antoine Laugier and Francesco Milizia. In the 19th century it was above all Viollet-le-Due, Gottfried Solver, Henri Labrouste and Julien Citadel who advocated a close and realizable relationship between form and function in architecture.

Louis Sullivan is considered the founder of 'modern' functionalism. In his 1896 essay 'The tall office building, artistically considered', he coined the maxim 'form follows function'. He was building on the thoughts of the sculptor Horatio Greenough, who had introduced the notion of a dialectic between form and function in objects such as a frigate, in which design considerations were dictated by exposure to extreme physical conditions. Although Sullivan drew parallels with 'circling eagles' and open apple blossom, his expression was soon restricted in meaning to scarcely more than 'naked functionality' in the view of Functionalism.

 Thus restricted, the concept of Functionalism was to be used as the slogan for the most varied directions in avant-garde architecture during the lint half of the loth century: from the romantic organic architecture of Wright to the classicist Rationalism of Mies van der Rohe, from the lively Expressionism of Mendelsohn to the severe monumentalism of Terragni, from the independent formal play of Hiring to the strong geometries of Le Corbusier. Sharper contrasts arc hardly imaginable, and the bitter dispute between Hiring and Le Corbusier alone makes evident the inappropriateness of such generalizing classification.

The matter is further complicated by the fact that in the architectural discussion of the 1920s, Rationalism and Functionalism were highly disputed as to both meaning and relationship. However, after Alberto Sartoris was persuaded by Le Corbusier to change the title of the book he had originally planned in 1932 to call Architettura razionale, it was published instead as Gli Elementi dell'architettura funzionale, thus, the concept of Functionalism entered everyday parlance as a synonym of or even a replacement for Rationalism. Hence, his meaning was restricted, and he thus aligned himself with that very architectural movement that was least functionalist.  If the term can still be used justly to describe the 'organic' houses by Hiring, which tried to attribute to each function its own specially formed corner, it is hardly also appropriate in relation to a building by Gropius or Mies van der Rohe. Indeed, function is practically the last factor which determined the eminently symbolic form of the Fagus Factory or the Barcelona Pavilion. Their implications are far more complex and the first aspect to be sacrificed is precisely that usefulness on which their reputation had been founded. With regard to the inadmissible conflation of Rationalism and Functionalism, the words of Le Corbusier, that great apologist of engineers, and admirer of the Bleriots, the Aquitania and the Bugattis, should not be forgotten: 'Architecture is the masterly, correct and magnificent play of masses brought together in light . . . cubes, cones, spheres, cylinders or pyramids are the great primary forms which light reveals to advantage ... [they] are beautiful forms, the most beautiful forms.’