Louis Henry Sullivan was an American architect, and has been called a "father of skyscrapers" and "father of modernism." He was an influential architect of the Chicago School, a mentor to Frank Lloyd Wright, and an inspiration to the Chicago group of architects who have come to be known as the Prairie School. Along with Wright and Henry Hobson Richardson, Sullivan is one of "the recognized trinity of American architecture." The phrase "form follows function" is attributed to him, although the idea was theorised by Viollet le Duc who considered that structure and function in architecture should be the sole determinants of form. In 1944, Sullivan was the second architect to posthumously receive the AIA Gold Medal.

The Chicago School refers to two architectural styles derived from the architecture of Chicago. In the history of architecture, the first Chicago School was a school of architects active in Chicago in the late 19th, and at the turn of the 20th century. They were among the first to promote the new technologies of steel-frame construction in commercial buildings, and developed a spatial aesthetic which co-evolved with, and then came to influence, parallel developments in European Modernism. Much of its early work is also known as Commercial Style.