Ludwig Mies van der Rohe was a pioneering German-American architect known for developing the International Style of architecture. Some key aspects of his work included an emphasis on simplicity, open floor plans, and exposing structural elements. Some of his most famous works that exemplified his principles were the Barcelona Pavilion, which featured glass walls and floors, and Crown Hall at IIT, which had a large open space defined by a minimal steel structure. Mies strove to create flexible, universal spaces using modern materials like steel and glass.