Functionalism is a philosophical, symbolic, social, and economic approach to architecture and design whereby form is arrived at through its intended function. It is often thought to be one of the two approaches to architecture and design in the 20th century, historicism being the other. The roots of Functionalism can be traced back to the beginning of design theory, particularly to Vitruvius, the Roman architect and engineer of the 1st century BC who asserted that the design of a structure should be determined by its use or function. Prominent followers of Vitruvius included 18th-century Rationalists Fra Carlo Lodoli, Marc-Antoine Laugier, and Francesco Milizia, and in the 19th century, Eugene-Emmanuel Viollet-le-Duc, Henri Labrouste, and Gottfried Semper.
With his motto ‘form follows function,’ American architect Louis Sullivan is considered the founder of 20th-century Functionalism. Functionalism became a label for an extremely wide variety of avant-garde architecture and design in the first half of the 20th century, including Ludwig Mies van der Rohe’s classical Rationalism, Erich Mendelsohn’s Expressionism, Giuseppe Terragni’s unadorned, heroic structures, Frank Lloyd Wright’s organic architecture, and Le Corbusier’s Cubist solids. There is no clear distinction between Rationalism and Functionalism, despite much debate on the subject.