Of the many architectural theories that were being discussed during the early years of the present century one of the most comprehensive and intriguing was the theory of Organic Architecture. This theory, as expounded by its two most prominent exponents — Louis H. Sullivan (1856–1924) and Frank Lloyd Wright (1869–1959)—was based on the concept of organic expression — a concept which, interpreted in architectural terms, incorporated many of the underlying principles of the more popular theories associated with the Modern Movement in Architecture — ‘functionalism’, ‘rationalism’, ‘expressionism’, etc.