When American architect Louis Sullivan coined the phrase "form follows function," he described what later became a dominant trend in Modernist architecture. Louis Sullivan and other architects were striving for "honest" approaches to building design that focused on functional efficiency. Functionalist architects believed that the ways buildings are used and the types of materials available should determine the design.
Of course, Louis Sullivan lavished his buildings with ornamental details that did not serve any functional purpose. The philosophy of functionalism was followed more closely by Bauhaus and International Style architects.
Toward the end of the 20th century, the term Functionalism was used to describe any practical structure that was quickly constructed for purely practical purposes without an eye for artistry. However, for Bauhaus and other early Fuctionalists, the concept was a liberating philosophy that freed architecture from frilly excesses of the past.
Designed by Louis I. Kahn, the Yale Center for British Art in New Haven, Connecticut is an example of Functionalism in architecture.