One of the forerunners of modern architecture was the firm Adler and Sullivan. I am a huge fan of this duo who I first learned about when living in Buffalo, New York. Dankmar Adler and Louis Sullivan worked together in the Chicago area from 1880-1895. At one point Frank Lloyd Wright worked as an assistant to the firm and was influenced by Adler and Sullivan. The genius of the team was the use of new materials and engineering in their designs.
Louis Sullivan later went on to work in Buffalo on the Guaranty or Prudential Building that I saw and influenced my love of architecture.