Form follows function is a principle associated with modern architecture and industrial design in the 20th century. The principle is that the shape of a building or object should be primarily based upon its intended function or purpose.

The authorship of the phrase is often, though wrongly, ascribed to the American sculptor Horatio Greenough, whose thinking to a large extent predates the later functionalist approach to architecture. It was, however, the American architect Louis Sullivan who coined the phrase, in 1896, in his article The Tall Office Building Artistically Considered. Here Sullivan actually said "form ever follows function", but the simpler (and less emphatic) phrase is the one usually remembered. For Sullivan this was distilled wisdom, an aesthetic credo, the single "rule that shall permit of no exception". The full quote is thus:

It is the pervading law of all things organic and inorganic,
Of all things physical and metaphysical,
Of all things human and all things super-human,
Of all true manifestations of the head,
Of the heart, of the soul,
That the life is recognizable in its expression,
That form ever follows function. This is the law.

Sullivan developed the shape of the tall steel skyscraper in late 19th Century Chicago at the very moment when technology, taste and economic forces converged violently and made it necessary to drop the established styles of the past. If the shape of the building wasn't going to be chosen out of the old pattern book something had to determine form, and according to Sullivan it was going to be the purpose of the building. It was "form follows function", as opposed to "form follows precedent". Sullivan's assistant Frank Lloyd Wright adopted and professed the same principle in slightly different form—perhaps because shaking off the old styles gave them more freedom and latitude.